Andaman Sea vs Gulf of Thailand: which is better for diving? Mantas, whale sharks and two monsoons

The Andaman Sea is the stronger choice for bigger-animal diving. Think granite boulders, deep pinnacles and liveaboards, with oceanic mantas regularly encountered at Koh Bon and Koh Tachai north of the Similan Islands and whale sharks reported in most seasons. The Gulf of Thailand is calmer and cheaper and suits beginners better. Koh Tao has some of the world's busiest dive training, plenty of turtles and barracuda, and its own chance of a whale shark at the offshore pinnacles.

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Picture one morning, two seas. On the west coast, a liveaboard is anchored off a stack of granite boulders the size of houses, and the first divers to roll in are already pointing at a dark diamond shape coming up out of the blue. Across the peninsula on the east coast, a longtail full of nervous open water students is putting out of a bay on Koh Tao. With luck, some of them will see their first hawksbill turtle, and every one of them will forget about their buoyancy while they do. Thailand has two coastlines on two different seas, and they have different monsoon seasons, so most of the year at least one of them is calm. That suits two kinds of diver. One is the experienced diver who wants manta cleaning stations and current-swept pinnacles. The other is the new diver, or the diver's sensible partner, who wants warm, easy water, cheap courses and a beach bar. If you already know you want to go west, you can compare Andaman boats and dates on LiveAboard.com. If you're still deciding, read on. This guide covers when to go, which coast suits you, and the region-by-region detail you need before you book.

When is the best time to dive Thailand? (Short answer: it depends which coast)

The key fact is that the two coasts get their monsoons at different times. That's why so many divers end up with a Thailand plan that moves between both coasts.

Andaman Sea: roughly October to May

The Andaman side has a clear high season, from around mid-October to mid-May. The Similan and Surin marine national parks close every year during the southwest monsoon, usually from about mid-May to mid-October. The southern pinnacles of Hin Daeng and Hin Muang also close over the wet months, and day boats from Koh Lanta stop going out there until the season reopens. Exact opening and closing dates can change from year to year, so check with the national parks or your operator before you book.

The later months are better for big animals. Whale sharks are most often reported from about February to April. Manta sightings at Koh Bon are reported right through the season, with February and March often mentioned as the best months.

Gulf of Thailand: diveable most of the year

Koh Tao and the Gulf islands are diveable almost all year. The main exception is the northeast monsoon, which usually brings a rougher spell from about November into December or January. In those weeks visibility can drop to single figures and the ferry crossing can test your breakfast. Even then, diving usually carries on, often with fewer divers around. Whale sharks are reported around Koh Tao at most times of year, with peaks most often mentioned from about March to May and again around September and October.

Water temperature and wetsuits

Both seas are warm, generally around 27 to 30 degrees C at the surface. In the Andaman you can drop through noticeably cooler water at depth, especially around March and April. That first cold layer is quite a shock when you've been diving in bathwater for a week. A 3mm wetsuit is the sensible default. A shorty is fine for one or two dives a day on Koh Tao, but if you're doing four dives a day for a week on a liveaboard, you'll be glad of full-length 3mm.

Insider tip: If your dates are fixed between late October and early December, pick a coast based on the weather rather than the brochure. That's when the Andaman is starting to settle and the Gulf is about to get its monsoon. In that window, the west is usually the safer bet.

Similan and Surin Islands: where Thailand keeps its mantas

This is the headline region for Thai diving and the reason most liveaboards exist. The Similan Islands sit roughly 65 to 70 km off the coast of Khao Lak. The granite boulders and swim-throughs on the west sides of the islands contrast with gentler sloping coral reefs on the east sides. Further north, the Surin area and Richelieu Rock offer some of the best mixes of macro and big animals in the country. Also in this northern stretch are Koh Bon and Koh Tachai, two outlying islands that are known as manta cleaning spots.

Conditions vary. The east-side reefs are generally relaxed, while the exposed boulders and pinnacles can have real current and surge. Visibility is generally good in the high season, though plankton can reduce it later on. Thermoclines come and go late in the season.

Who it suits: Open water divers can manage many sites with a good guide. Advanced certification and a few dozen dives will get you much more out of the deeper pinnacles and current dives.

How to get there: Mostly by liveaboard from Khao Lak or Phuket, usually for several nights. There are speedboat day trips from Khao Lak to the Similans, but Richelieu Rock and the northern sites are much easier to dive from a boat that stays out overnight. You can see the routes on LiveAboard.com's Similan page.

Commonly encountered:

• Oceanic manta (Mobula birostris): at Koh Bon and Koh Tachai, mantas come up out of the blue to hover over cleaning stations while small wrasse pick them over. They are oceanic mantas, not the reef mantas you may know from the Maldives or Indonesia. Since 1995, photo-ID records have logged more than 300 individual oceanic mantas in the Similan-Surin area.
• Whale shark (Rhincodon typus): reported in most seasons around Richelieu Rock and the northern sites, most often from about February to April.
• Harlequin shrimp (Hymenocera picta) and seahorses (Hippocampus spp.): the reason the photographers on your boat come up from Richelieu Rock with 400 frames of one tiny animal and no regrets.

Insider tip: If you see a manta, the best thing you can do is stay still and low beside the cleaning station rather than swim toward it. Divers who chase a manta usually see the back of it. Divers who keep still, breathe slowly and make big, gentle bubbles often get repeat passes. Your air will also last longer, and the divemaster will notice.

Phuket, Phi Phi and Krabi: the easygoing middle

This is where most people do their first Andaman dive, often after a few days of looking at the sea from a sunbed. Day boats from Phuket head to the Racha islands, to the Phi Phi archipelago and to offshore rocks like Shark Point and Anemone Reef. Krabi and Ao Nang boats work the limestone islands and the Phi Phi area from the other side.

Sites are generally shallower and more sheltered than the offshore pinnacles. Visibility is more variable, especially near the mainland, and changes with tides and plankton. The reward is how easy it is to get to. You can dive in the morning and be back at your hotel for lunch.

Who it suits: Beginners, people doing courses, families, snorkelers and anyone who wants to fit a few dives into a beach holiday. It's also a useful warm-up before a liveaboard.

How to get there: Day boats from Phuket, Phi Phi, Ao Nang and Krabi. Some of the shorter liveaboard itineraries also include Phi Phi.

Commonly encountered:

• Blacktip reef shark (Carcharhinus melanopterus): commonly seen cruising the shallows around Phi Phi, often early in the morning. Snorkelers sometimes see them before divers do, which divers find quietly annoying.
• Leopard shark (Stegostoma tigrinum): sometimes found resting on sand near the offshore rocks east of Phuket, such as Shark Point and Anemone Reef, looking about as threatening as a large, spotty sofa cushion. Numbers have declined, and since 2025 a rewilding project has been releasing young leopard sharks off Phuket. It was, as your guide will explain, definitely here yesterday.
• Hawksbill turtle (Eretmochelys imbricata): grazing on reefs around the islands.

Insider tip: On day trips, ask your shop which way the boat is going before you book. On a calm day the offshore rocks are some of the best dives in this area. On a windy day, a boat to the sheltered Racha or Phi Phi sites is the more comfortable choice, and your stomach will agree.

Koh Lanta and the southern pinnacles: deep purple and big surprises

Off Koh Lanta, two isolated pinnacles rise out of open sea: Hin Daeng and Hin Muang. They're roughly 50 to 70 km from Lanta, so getting there is a full day out. Hin Muang drops steeply and is covered in purple soft coral. Hin Daeng is generally described as a little shallower. Closer to Lanta, the Koh Haa islands have caverns, swim-throughs and calmer, shallower diving.

The pinnacles are open water, so expect current, some surge and blue water on every side. The Koh Haa sites are more sheltered and easier.

Who it suits: Advanced divers who are comfortable in current and in open blue water should dive the pinnacles. Koh Haa is fine for newer divers in calm conditions.

How to get there: Long day trips by speedboat from Koh Lanta, or on southern liveaboard routes that run from Phuket to Hin Daeng. See which boats include the pinnacles on LiveAboard.com.

Commonly encountered:

• Whale shark (Rhincodon typus): the pinnacles are one of the Andaman's better places for a whale shark, with sightings reported mainly from February to April. A whale shark passing a pinnacle covered in purple soft coral is the kind of sight you never forget.
• Oceanic manta (Mobula birostris): Hin Daeng and Hin Muang are among the main manta sites tracked by the Thailand Manta Project.
• Soft corals (Dendronephthya spp.): the purple and pink colour that Hin Muang is famous for. Unlike the sharks, they're always there.

Insider tip: These are open-water pinnacles a long way from anywhere, so bring a surface marker buoy (SMB) and know how to deploy it before you get there. The boat crew will thank you, and you will thank yourself if the current pushes you off the rock.

Koh Tao and the Gulf islands: the world's busiest classroom

Koh Tao is a small island that certifies huge numbers of divers every year, and it's easy to see why. The bays are sheltered, the water is warm, the reefs are shallow and the island's whole economy is diving. Koh Phangan and Koh Samui are nearby. Between the islands sit the Gulf's two best-known offshore sites: Chumphon Pinnacle, north-west of Koh Tao, and Sail Rock, between Koh Tao and Koh Phangan. Sail Rock is famous for its vertical swim-through, known as the Chimney.

The Gulf is shallower than the Andaman, so visibility is usually lower and depends more on weather and plankton. On a good day it's lovely. Most bay sites are gentle, with little current. The offshore pinnacles have more current and more fish.

Who it suits: Anyone learning to dive, anyone getting back into it after a break, and anyone travelling in the Andaman off-season. The pinnacles also give experienced divers a reason to come.

How to get there: Almost entirely land-based. Dive shops run morning and afternoon day boats from the island.

Commonly encountered:

• Chevron barracuda (Sphyraena putnamae): big, slowly turning schools at Chumphon Pinnacle and Sail Rock. Swimming into the middle of one is one of the best experiences in Thailand.
• Whale shark (Rhincodon typus): reported at the offshore pinnacles, with peaks often mentioned from about March to May and again around September and October. If a whale shark turns up, every boat on the island tends to change plans.
• Batfish (Platax spp.) and hawksbill turtles (Eretmochelys imbricata): batfish hang curiously in mid-water off the pinnacles, while the turtles graze on the shallow reefs and patiently ignore the latest group of students.
• Christmas tree worms (Spirobranchus giganteus): common on many of the island's sites, and they pull back into their tubes the moment a student flaps a fin near them.

Insider tip: On Koh Tao, ask for the earliest boat. Sites get busy by mid-morning, and an early start gives you calmer water and better visibility before the training groups arrive and stir up the sand.

What marine life will you see in Thailand?

Big animals

Thailand's big-animal headliners are the oceanic manta (Mobula birostris) and whale shark (Rhincodon typus). Mantas are mostly an Andaman encounter. The Thailand Manta Project notes they've been sighted in the Gulf but are seen far more often on the Andaman side. Whale sharks are reported on both coasts, at different times of year. Blacktip reef sharks (Carcharhinus melanopterus) are the shark most visitors see, and the leopard shark (Stegostoma tigrinum) is a much-loved bonus. Like all wildlife, none of these are guaranteed.

Reef life

Expect schools of chevron barracuda (Sphyraena putnamae), batfish (Platax spp.), groupers, snappers and fusiliers on the pinnacles. Hawksbill turtles (Eretmochelys imbricata) feed on the reefs, and moray eels look out of the cracks. Of the two coasts, the Andaman has the bigger and more dramatic hard and soft corals and sea fans. The Gulf's anemone-covered pinnacles make up for it in colour.

Macro

Richelieu Rock is Thailand's best-known macro site, with harlequin shrimp (Hymenocera picta) and seahorses (Hippocampus spp.) commonly reported, and ornate ghost pipefish (Solenostomus paradoxus) also recorded there. Both coasts also have plenty of nudibranchs, shrimps and small crabs for anyone with a torch and patience.

Liveaboard or land-based?

Which one you choose mostly depends on which coast you're diving.

Liveaboard (mainly Andaman): The only practical way to properly dive the Similans, Koh Bon, Koh Tachai and Richelieu Rock in one trip. Typical trips run for several nights from Khao Lak or Phuket. You'll do up to four dives a day, including some early mornings and a night dive, and you won't spend hours every day travelling to and from the sites. The downsides are a fixed schedule, less time on land and, for some people, the swell on the crossing. It suits divers who want the best of the Andaman in a short time. You can compare routes, lengths and dates on LiveAboard.com, which is the easiest way to see everything that's running in one place.

Land-based (both coasts): Phuket, Krabi, Koh Lanta and Koh Tao all have plenty of day-boat diving. It's flexible and usually cheaper, and good for mixed groups where not everyone dives. The trade-off is longer boat rides for less time underwater on the far sites, and you'll mostly miss the remote northern Andaman. It suits beginners, families and anyone combining diving with a holiday. A lot of experienced divers do both: a few days on Koh Tao or Phuket to get comfortable, then a liveaboard.

How do you get to Thailand's dive regions?

Most international visitors arrive through Bangkok (Suvarnabhumi), and many also fly direct into Phuket.

• Similans and Surin: Fly to Phuket. Liveaboards leave from Phuket or from Khao Lak, about an hour and a half north of Phuket airport by road. Many operators arrange transfers.
• Phuket, Phi Phi and Krabi: Fly to Phuket or Krabi, then take day boats or the ferries between the islands.
• Koh Lanta: Fly to Krabi, then travel on by road and car ferry.
• Koh Tao: Fly to Koh Samui and take a ferry, or travel to Chumphon or Surat Thani on the mainland and catch a ferry from there.

Entry requirements for Thailand change, so check the current rules for your passport before you book anything.

Planning tips

• Certification and experience: Open Water is enough for Koh Tao, Phuket and the easier Similan sites. Advanced certification, plus some current experience, makes the pinnacles (Hin Daeng, Hin Muang, Chumphon Pinnacle, Sail Rock and Richelieu Rock) much more enjoyable. Some liveaboards ask for a minimum number of logged dives, so check before you book.
• Nitrox: Widely offered on Andaman liveaboards and well worth it if you're doing four dives a day. Get certified before you go, or ask whether the boat can run the course on board.
• Dive insurance: Get proper dive-specific cover. The northern Andaman sites are a long way from shore.
• Gear that matters: An SMB and a reel or spool are essential for pinnacles in open water. A torch makes a big difference for macro, for looking into cracks and for night dives. A reef hook is rarely needed, but some divers like one for a few current-swept pinnacles. Use it only on bare rock, never on coral, and follow your guide's advice.
• Flying after diving: Wait at least 18 hours after repetitive dives before you fly. Put a beach or temple day at the end of your liveaboard.
• Park fees: The Similan, Surin and Lanta marine parks charge entry fees. Your operator will usually arrange them or tell you how to pay.

Booking your trip

Once you've chosen a coast and a month, the next step is finding a boat or dive base that fits your dates and experience. You can compare current Thailand trips on LiveAboard.com.

Why I recommend LiveAboard.com

I've worked on liveaboards for 13 years. I've booked through LiveAboard.com many times, and so have a lot of my guests. They don't sponsor Fins and they don't see what we write. Here's why I point people to them:

• The biggest choice of boats. In most of the places we dive, they list more boats and departure dates than anyone else, so you can compare everything in one place.
• You don't pay more. Booking through them doesn't cost more than booking with the boat directly, and you get some of the most competitive prices around.
• Service that actually helps. They answer questions quickly and help with the planning before your trip and with anything that comes up after it.

If you're not sure which boat or itinerary is right for you, they're a great place to start: compare liveaboards in Thailand.

Before you go: get Fins

Know what you're looking at. Fins is a marine life ID app built by divers. Before your trip, use it to get to know the animals you're likely to meet. After each dive, identify what you saw from a photo, or answer a few simple questions if you didn't get a shot, and keep it all in your dive log. Download Fins before you fly.

For Thailand, learn the difference between a manta's markings and a mobula's flapping silhouette, and spend a few minutes on Richelieu Rock's macro animals. That way the harlequin shrimp you photograph doesn't end up filed in your camera roll as 'orange thing, possibly a crab'.

FAQ

Is the Andaman Sea or the Gulf of Thailand better for diving?

For big animals and dramatic topography, go to the Andaman Sea, especially the Similans and Richelieu Rock. For cheap, easy training and diving almost all year, go to the Gulf around Koh Tao. Many divers combine the two.

When are the Similan Islands closed?

The Similan and Surin marine national parks usually close from about mid-May to mid-October during the southwest monsoon. Exact dates change from year to year, so check with the park authorities or your operator.

Where can you see whale sharks in Thailand?

Whale sharks are reported at Richelieu Rock and the Hin Daeng and Hin Muang pinnacles in the Andaman, mostly from about February to April. In the Gulf, they're reported at Chumphon Pinnacle and Sail Rock, with peaks often mentioned around March to May and again around September and October. Sightings are never guaranteed.

Can beginners dive in Thailand?

Yes. Koh Tao is one of the most popular places in the world to learn, and Phuket, Phi Phi and Krabi have plenty of sheltered, shallow sites. Leave the open-water pinnacles until you have some experience in current.

What wetsuit do I need for diving in Thailand?

Water is generally 27 to 30 degrees C, so 3mm is ideal. A shorty is fine for a couple of dives a day, but on a liveaboard with four dives a day, a full-length 3mm keeps you warmer, especially in an Andaman thermocline.

Do I need a liveaboard to dive the Similans?

Not strictly. Day trips run from Khao Lak, but a liveaboard is the practical way to reach Koh Bon, Koh Tachai and Richelieu Rock and to dive them at their best.

Two seas, two monsoons and a whole country of good diving between them. Whichever coast you choose, you'll get more out of it if you know what you're looking at. Download Fins before you fly, and spend your surface intervals identifying what you saw instead of arguing about it.