The Best Dive Sites in Thailand: Similans, Richelieu Rock, Sail Rock and More

By Bex Callan | Checked by Anan Wattana, TAT-licensed professional guide

Published April 8, 2026

Key takeaways

Thailand's headline sites split by coast: the Similan Islands, Richelieu Rock and the Hin Daeng and Hin Muang pinnacles on the Andaman side; Sail Rock and Chumphon Pinnacle in the Gulf.
The Andaman sites are best roughly mid-October to mid-May, and the Similan National Park closes to all visitors in the low season, about mid-May to mid-October.
The Gulf sites dive year-round, with the poorest conditions usually around October to December.
Whale sharks and manta rays are seen most often at Richelieu Rock in the Andaman and at Sail Rock in the Gulf, though never on any guaranteed schedule.
The best sites carry depth and current and often sit far from a decompression chamber, so match them to your certification and carry diving-specific insurance.

Thailand’s best diving is a short list of world-class pinnacles and reefs split across two coasts: the Similan Islands, Richelieu Rock and the Hin Daeng and Hin Muang pinnacles on the Andaman side, and Sail Rock and Chumphon Pinnacle in the Gulf. Over eight years of season-chasing I dived most of them more times than I can count, and each one earns its reputation for a different reason. Here is what each site is actually for, when to go, and where the big animals show up.

Richelieu Rock: the Andaman’s crown jewel

Richelieu Rock is the site most experienced divers in Thailand name first, a horseshoe-shaped limestone pinnacle in the Andaman Sea, famous for its purple soft coral, dense macro life and the country’s most reliable whale shark encounters ¹. It sits well offshore near the Surin Islands, so most people reach it on a liveaboard rather than a day boat.

It is best roughly mid-October to mid-May, in line with the whole Andaman season ². Whale sharks and manta rays are seen here more often than anywhere else in Thailand, though “often” is relative and never guaranteed ¹. My best-ever dive was here on an April afternoon when a whale shark the length of a minibus swept through the group twice; my worst was the same site a week later in a current so strong I finned hard the whole dive and saw almost nothing. That is Richelieu: high reward, real conditions. It carries depth and current, so it suits a comfortable, ideally Advanced diver.

The Similan Islands: granite scenery and easy reefs

The Similan Islands are Thailand’s signature dive destination, a chain of granite islands whose sites range from dramatic boulder swim-throughs on the west to gentle sloping coral reefs on the east ¹. The variety is the point: there is something here for a nervous newer diver and something for a jaded veteran, often on the same day.

The crucial fact is access. The Similan National Park closes to all visitors in the low season, about mid-May to mid-October, every year, so you can only dive here roughly mid-October to mid-May, with peak visibility usually December to April ². The park also charges a foreigner entry fee, so carry cash. I learned the closure the hard way, arriving in Khao Lak in July to find every Similan boat beached for the season. Check your dates against the park calendar before you commit. How the parks and seasons interlock is covered in the diving in Thailand pillar guide.

Hin Daeng and Hin Muang: the deep-water thrill dives

Hin Daeng (“Red Rock”) and Hin Muang (“Purple Rock”) are the Andaman’s advanced pinnacles, dropping in dramatic walls well past recreational limits and swept by current that draws big pelagic life ¹. Hin Muang holds one of Thailand’s deepest visible walls, and both sites see manta rays and, in season, whale sharks.

These are not beginner dives. The combination of depth, current and distance from help means they suit experienced divers, ideally Advanced certified and comfortable in blue water ³. They dive in the same mid-October to mid-May Andaman window. Treat them as the reward for building experience on gentler sites first, not the place to test a new card. Matching a site to your training is the whole message of the learn to dive on Koh Tao guide.

Sail Rock: the Gulf’s year-round pinnacle

Sail Rock (Hin Bai) is the best dive site in the Gulf of Thailand, a granite pinnacle rising from the seabed to just above the surface between Koh Tao and Koh Phangan, split by a famous vertical swim-through chimney ¹. It gathers schooling barracuda and trevally, and it is the Gulf’s most reliable spot for whale sharks and the occasional manta ¹.

Its great virtue is timing: the Gulf dives year-round, with only the October to December stretch typically poor, so Sail Rock is diveable when the Andaman season is shut ¹. That chimney was my first real “why people do this” dive, ascending inside the rock with silver fish stacked overhead, and it is still the site I send every certified visitor to Koh Tao toward.

Chumphon Pinnacle: Koh Tao’s big-fish site

Chumphon Pinnacle is Koh Tao’s premier site, a large granite pinnacle a short boat ride northwest of the island, topping out around 14 metres and dropping well below, known for grey reef sharks, schooling fish and the occasional whale shark ¹. It is the Gulf site that most feels like the “big” diving of the Andaman.

Like the rest of the Gulf it dives year-round, poorest around October to December ¹. It carries more depth and current than the shallow training bays, so it is a natural first “proper” site once you have a few dozen dives, and a place many people log their Advanced course. As always, mind the chamber map: the nearest decompression chambers are on Koh Tao, Koh Samui and Phuket, so dive conservatively and carry cover that includes a chamber ride ³. What that cover must include is set out in the travel insurance for adventure guide.

General information, not diving instruction. Conditions, visibility and park access change by season and year; always dive within your training and current certification, follow your dive operator, and carry proper diving insurance.

Common questions

What is the best dive site in Thailand?

Richelieu Rock is the one most experienced divers name first: a horseshoe-shaped pinnacle in the Andaman, dense with soft coral and macro life, and the country's most reliable whale shark site. But 'best' depends on what you want. The Similan Islands offer dramatic granite scenery, Sail Rock in the Gulf is a superb year-round pinnacle, and Hin Daeng and Hin Muang are the deep-water thrill dives. There is no single winner.

When can you dive the Similan Islands?

Roughly mid-October to mid-May. The Similan National Park closes to all visitors in the low season, about mid-May to mid-October, every year to let the reefs recover, so liveaboards and day trips simply do not run then. Peak visibility tends to fall in the middle of the open season, so aim for the December to April window if you can.

Where can you see whale sharks in Thailand?

Most often at Richelieu Rock on the Andaman coast and at Sail Rock in the Gulf. Both are seasonal and never guaranteed, because whale sharks are wild and wander, but those two pinnacles are where sightings cluster, usually in the clearer-water months of each coast's season. Manta rays turn up at the same sites.

Do I need a liveaboard to dive the Similans?

Not strictly, but it is the best way. The Similan Islands and Richelieu Rock sit well offshore, so day trips are long and rushed. A multi-day liveaboard puts you on the sites at the best times of day and lets you reach Richelieu Rock comfortably. Day trips exist and are cheaper, but you spend a lot of the day on a boat.

Are Hin Daeng and Hin Muang suitable for beginners?

Generally no. Hin Daeng and Hin Muang are deep-water pinnacles in the Andaman with strong current and dramatic walls dropping well below recreational limits, so they suit experienced, comfortable divers, ideally Advanced certified. They are spectacular, but they are not the place to log your fifth dive. Build experience on gentler sites first.

Which coast has the better diving, the Gulf or the Andaman?

The Andaman has the famous big-fish and big-scenery sites and clearer high-season visibility; the Gulf has cheaper, year-round diving and the excellent Sail Rock. If you want the postcard sites and can travel in the Andaman season, go west. If you want flexibility in almost any month, the Gulf delivers. Many people do both on one trip.

References

1.: Diving and Snorkelling in Thailand, Tourism Authority of Thailand. ↩
2.: Marine National Parks of Thailand, Department of National Parks, Thailand. ↩
3.: Diving Safety and Health Resources, Divers Alert Network. ↩

Written by Bex Callan. Checked by Anan Wattana, TAT-licensed professional guide.

Our guides are written from personal experience and reviewed by a long-term Thailand resident for accuracy. Read our editorial policy.