Three pinnacles in the northern Andaman Sea — Koh Bon, Koh Tachai and Richelieu Rock — make up the single best stretch of big-animal diving in Thailand. This is where oceanic manta rays circle cleaning stations and whale sharks drift through clouds of fusiliers. All three sit beyond the Similan islands and are reached by liveaboard, so this guide covers when to go, what depths and dive levels to expect at each site, and how to choose the right boat.
When to go: the Andaman dive season
The Andaman dive season runs from mid-October to mid-May, fixed each year by the closure of Mu Ko Similan National Park. The park reopened on 15 October 2025 and is scheduled to close on 15 May 2026, with the next season expected to open around 15 October 2026. Outside these dates the sites are off-limits and the boats do not run.
Conditions are not uniform across the season. December to March brings the calmest seas and the clearest water, ideal for first-timers to the region. The big pelagics, however, peak later: whale sharks and manta rays concentrate at Richelieu Rock, Koh Bon and Koh Tachai toward the end of the season, when plankton-rich currents draw them in. If a manta encounter is your priority, late season is the smart bet — though sightings are never guaranteed.
Koh Bon Pinnacle: oceanic manta hotspot
Koh Bon is the first true manta site north of the Similans. The main attraction is a submerged pinnacle off the island's West Ridge, with its summit at roughly 18 metres dropping beyond 40 metres into the blue. Oceanic mantas visit the cleaning stations here from around January to April, sometimes hanging motionless overhead while cleaner wrasse work their wings.
This is an Advanced-level dive. Currents are often strong, especially around the full and new moon, and depth management matters. Best window: January to April. Expect leopard sharks resting on the sand, barracuda, tuna and the occasional whale shark cruising past the ridge.
Koh Tachai Pinnacle: the current-swept arena
Further north, Koh Tachai Pinnacle is one of the Andaman's most exhilarating dives. A pair of dome-shaped granite pinnacles tops out around 12 to 18 metres and drops to about 45 metres, draped in colourful gorgonian sea fans. Strong currents funnel nutrients across the site, pulling in big pelagics and enormous schooling fish.
Because of those currents, descents are made on the buoy line and the site is reserved for Advanced divers. The best months are February to April. The reward is a genuine big-animal arena — manta rays, whale sharks, walls of barracuda, batfish, trevally and dogtooth tuna all work the current.
Richelieu Rock: Thailand's number-one dive site
Richelieu Rock, in the Mu Ko Surin park to the northeast, is the crown jewel of any Andaman liveaboard and consistently rated Thailand's top dive site. It is a solitary horseshoe-shaped pinnacle rising from about 35 metres to just below the surface, blanketed in vivid purple soft coral.
The site combines world-class macro — seahorses, frogfish, cuttlefish — with the chance of large pelagics, and it is the most reliable spot in Thailand for seasonal whale sharks. Depths run 5 to 35 metres, suiting Intermediate to Advanced divers; currents can pick up, so comfort with multi-level profiles helps. Best months are February to April.
Do I have the right level?
Koh Bon and Koh Tachai are best for Advanced Open Water divers with roughly 20 to 50 logged dives, because of the depth and current. Richelieu Rock suits confident Intermediate divers and up. If you hold an Open Water certification, most liveaboards let you join and complete an Advanced course on board, which unlocks the deeper sites — ask when you book.
Even strong divers should plan for open-water crossings to these offshore pinnacles, which can get choppy. If you are prone to seasickness, bring your remedy, choose a lower mid-ship cabin, and take it before sailing.
Getting there: choosing a liveaboard
All three sites lie beyond comfortable day-trip range, so a liveaboard is the standard way to dive them, typically on 3 to 5 night Similan and Surin itineraries that combine Koh Bon, Koh Tachai and Richelieu Rock in one trip. Departures run weekly per vessel and near-daily across the wider fleet during the season.
Our top picks for this route are MV Pawara and MV Sawasdee Fasai — well-rated, comfortable boats that run the classic northern-Andaman circuit. For a strong mid-range option, the South Siam vessels (South Siam 3 and 4) and Deep Andaman Queen cover the same sites, and the Manta Queen fleet offers some of the most flexible scheduling. Berths are sold at operator rate parity, so you never pay more than booking direct. Cabin pricing varies by boat, season and itinerary length; contact us for current availability and a quote.
One budgeting note: Similan and Surin national park fees are almost never included and are paid separately in Thai baht cash. A multi-park itinerary that takes in both Mu Ko Similan and Mu Ko Surin (for Richelieu Rock) incurs fees for both parks.
Featured vessels for this trip
Frequently asked questions
When is the best time to see manta rays at Koh Bon?
Oceanic mantas visit the cleaning stations at Koh Bon Pinnacle from roughly January to April. Late season generally gives the best odds across Koh Bon, Koh Tachai and Richelieu Rock, but sightings are never guaranteed.
What certification do I need to dive Koh Bon and Koh Tachai?
Both are Advanced-level sites due to depth and current, best suited to Advanced Open Water divers with around 20 or more logged dives. With an Open Water certification you can usually complete an Advanced course on board to unlock the deeper dives.
How deep are these dive sites?
Koh Bon Pinnacle starts near 18 metres and drops beyond 40. Koh Tachai Pinnacle runs about 12 to 45 metres. Richelieu Rock spans 5 to 35 metres, with most of the action in the shallower range.
When does the Andaman dive season run?
From mid-October to mid-May, set by the Mu Ko Similan National Park closure. The park reopened on 15 October 2025 and closes on 15 May 2026. December to March has the calmest seas; big pelagics peak later in the season.
Can I dive all three sites on one trip?
Yes. Standard 3 to 5 night Similan and Surin liveaboard itineraries combine Koh Bon, Koh Tachai and Richelieu Rock. Boats such as MV Pawara and MV Sawasdee Fasai run this northern-Andaman route during the season.
Are national park fees included in the price?
Almost never. Similan and Surin park fees are paid separately in Thai baht cash on the boat. Trips visiting both Mu Ko Similan and Mu Ko Surin (for Richelieu Rock) incur fees for both parks.