Thailand’s reputation for diving is well established, but it tends to be flattened by mass-market liveaboard operations that run the same north Andaman circuit November through April, slotting forty divers into identical itineraries on identical schedules. The sites are world-class. The format is not.
A private charter yacht on the Andaman Sea changes the geometry entirely. The same sites become yours on your own timetable - Richelieu Rock before the liveaboard fleet anchors, the Similan Islands at dawn, Hin Daeng on a day when the current has swung in your favour. The yacht stays where the diving is good rather than moving because the schedule says to. And north of the established circuit, the Mergui Archipelago stretches into Burmese waters with 800 islands and almost no traffic - territory that requires a self-sufficient vessel and the willingness to go where the grid runs out.
This is what a serious diving charter in the Andaman Sea looks like.
The Similan Islands
The Mu Ko Similan National Marine Park - nine granite islands running north to south roughly 65 kilometres off Khao Lak - is the anchor of Andaman Sea diving. The park closes in the monsoon season and reopens around 15 October each year, which concentrates the entire dive season into a window of roughly seven months. In that time it receives heavy traffic from Phuket and Khao Lak liveaboards, but the archipelago is large enough that a private charter can position away from the commercial fleet and find the headline sites in relative quiet.
The Similan Islands divide cleanly by coastline type. The eastern sides of the islands are generally gentler dives: sloping hard coral reefs running from 5 metres down to sandy bottoms at 25 to 30 metres, abundant table corals and staghorn formations, dense populations of reef fish. Breakfast Bend on the eastern side of Island 9 (Koh Ba Ngu) is a long drift reef that produces leopard shark sightings regularly and suits divers from beginner upwards. Anita’s Reef, running between Islands 5 and 6, is similar in character - excellent macro, turtles on the shallow reef top, and a broad depth range that works for multi-level dive planning.
The western sides are categorically different. The same granite geology that gives the Similans their surface appearance - rounded boulders stacked above the water like a collapsed wall - replicates itself underwater into a landscape of tunnels, arches, swim-throughs, and caverns. West of Eden on Island 7 (Koh Pa-Yu) is the standard introduction to this world: huge boulders covered in gorgonian fans and soft corals, channels where giant trevally hunt, and depth range from 5 to 40 metres. The currents on the western sides can be strong - particularly around full and new moons - which adds technical interest for experienced divers and keeps the less certain visitors on the eastern sites.
Elephant Head Rock (Hin Pusa) is a separate structure between Islands 7 and 8, three boulders breaking the surface that form an underwater playground of maze-like complexity - swim-throughs, sheer walls, a sand floor at 40 metres with whitetip reef sharks resting on it. Many experienced divers consider it the finest deep boulder dive in the archipelago. It requires a free descent - there are no mooring lines - and the current exposure means conditions can shift quickly. It is a dive that benefits from arriving early in the morning before the liveaboard tenders make the transit.
Christmas Point, on the northernmost tip of Island 9, is the other headline boulder site: a cluster of submerged granite at depths from 10 to 40 metres, walls carpeted with soft coral and sea fans, reliable manta ray sightings on the right tidal window, and a famous swim-through at 24 metres that earns consistent mention in dive guides.
The Similan Islands’ marine park is officially open 15 October to 15 May. The northern end of this window - October, November - sees calm water but occasionally reduced visibility as the season settles. December through February gives the most reliable combination of flat seas, 20 to 30-metre visibility, and water temperatures around 28 to 30°C. The whale shark peak runs February through May; mantas are present throughout the season.
Richelieu Rock
Roughly 18 kilometres east of Ko Surin Tai and administered as part of the Mu Ko Surin National Park, Richelieu Rock sits 45 kilometres offshore with no island in the immediate vicinity. It is a submerged horseshoe-shaped limestone pinnacle that breaks the surface only at low tide, surrounded by open water, and is consistently voted the single best dive site in Thailand. The offshore position is significant: it means no day-trip boat from the mainland can reach it without a lengthy run, and commercial liveaboard traffic, while present, cycles through on schedules that leave gaps.
The reason for the site’s reputation is simple: nutrient-rich upwellings from the deep water surrounding the pinnacle feed a planktonic soup that draws everything up the food chain. The purple soft corals and sea fans that cover the rock’s surface give it its colour signature. The density of marine life above that substrate - ghost pipefish in the sea fans, seahorses, harlequin shrimp in the shallower sections - makes it a macro photographer’s defining site. And the pelagic visitors that the plankton blooms attract are what put it on the international map: whale sharks arrive regularly from February through May, mantas appear throughout the season, and the schools of chevron barracuda, giant trevally, and tuna that circle the deeper sections give every dive a sense of the open ocean pressing in.
Richelieu is worth multiple dives on any visit. The site is large enough to cover different sections on each entry, and conditions - current direction, visibility, what is feeding at what depth - change considerably across a day. The standard liveaboard approach gives divers two or three dives here before moving on. A private charter can anchor or pick up a park mooring buoy and spend a full day on the site, diving morning, afternoon, and a night dive if conditions allow. Night diving Richelieu - when the macro life that hides during daylight hours emerges and the colour of the soft corals under torch light becomes extraordinary - is one of the experiences that distinguishes a charter from a liveaboard itinerary.
The current at Richelieu can be strong and somewhat unpredictable, which makes it more appropriate for intermediate to advanced divers. A divemaster or guide with personal experience at the site is worth having for the first dive.
Koh Bon and Koh Tachai
Two islands that most Andaman liveaboards pass through en route to Richelieu, and that deserve more attention than the transit treatment they typically receive.
Koh Bon, the southernmost of the Surin/Similan northern chain, is Thailand’s most reliable site for manta ray encounters. A prominent rocky ridge runs from the island’s west point, and on the right tidal window - incoming current from the south, water temperature around 28°C - the mantas arrive to feed on the plankton concentrations above the ridge. Two mantas feeding in open water above a diver at 15 metres is a reasonably common experience here in season. The same site also produces Napoleon wrasse, large grouper, and - at depth off the ridge’s western extension - pelagic encounters with the open blue.
Koh Tachai was temporarily closed to all visitors for several years due to tourist damage and has progressively reopened. The Koh Tachai Plateau - a submerged plateau of hard corals and granite boulders - is the principal dive site, with the same pelagic character as Richelieu: schooling barracuda, tuna, and whale sharks that pass through when the seasonal conditions align. For a charter yacht willing to run the additional distance north of the Similans, both Koh Bon and Koh Tachai offer diving that a purely Similan-focused itinerary misses.
Hin Daeng and Hin Muang
The southern Andaman circuit, accessed primarily from Phuket and operating on a different schedule to the northern liveaboards, centres on two underwater pinnacles roughly 80 kilometres south of Phuket that hold some of the most intense pelagic diving in Southeast Asia.
Hin Muang (Purple Rock) is an entirely submerged pinnacle - it never breaks the surface - that drops from around 8 metres to over 60 metres. The vertical walls are dense with purple soft corals and sea fans (the colours give both pinnacles their names), and the currents that sweep around the structure bring the nutrients that keep this ecosystem functioning at the level it does. Whale sharks appear here regularly, along with large aggregations of manta rays - Hin Muang and the adjacent Hin Daeng are considered among the most reliable manta sites in the region outside the Maldives. The currents are strong and variable; this is a site for experienced divers who are comfortable in current and with drift diving technique.
Hin Daeng (Red Rock) breaks the surface, giving visiting boats a reference point. It shares Hin Muang’s character - vertical walls, sea fans, strong currents, pelagic visitors - but adds more pronounced topographic variety, with caves, overhangs, and deeper penetration dive options. The two sites are typically dived together on the same visit, with the itinerary depending on current direction and what has been sighted on recent dives. When both sites are running well on the same day - mantas at Hin Daeng in the morning, whale sharks circling Hin Muang in the afternoon - it is the kind of diving day that forms the reference point for everything that follows.
Both sites are accessible from a charter yacht based in Phuket or the Phi Phi Islands. The transit is longer than from Khao Lak, but a charter operating from Phuket can combine Hin Daeng and Hin Muang with Koh Haa (five smaller islands south of Koh Lanta with excellent sheltered reef diving and consistent leopard shark sightings) into a southern Andaman circuit that most liveaboards do not run.
The Mergui Archipelago
North of Richelieu Rock, past the Thai border, the Mergui Archipelago extends 400 kilometres through Burmese territorial waters. Around 800 islands, largely uninhabited, connected by diving that has been accessible to foreign liveaboards only since the late 1990s and that remains some of the most pristine reef in the Andaman Sea. The relative difficulty of access - Myanmar authorities require boats to obtain specific permits and transits to involve a Burmese government representative or approved guide in some cases - keeps traffic low. Which is the point.
The Burma Banks - three submerged seamounts called Silvertip Bank, Rainbow Bank, and Big Bank, roughly 110 kilometres north of the Surin Islands - are the most discussed dive destination in the Mergui. The silvertip sharks for which Silvertip Bank is named are present in numbers significant enough that this is one of the few locations on earth where entering the water virtually guarantees shark encounters: silvertips, oceanic whitetips, and nurse sharks are all documented. The deeper sections attract pelagic species that are now rare elsewhere in the region. The surface conditions in the Burma Banks require an ocean-capable vessel - the seamounts sit in open water with no protection from swell - which is precisely the kind of vessel a properly equipped charter yacht represents.
The rest of the Mergui Archipelago is less focused on single headline sites and more about the cumulative effect of diving unexplored reefs over two weeks without seeing another boat. Islands like Lampi and Black Rock, the passages around Western Rocky, and the reef systems of the Three Islets hold healthy coral and large marine life in densities that the Thai national parks, with their managed visitor numbers, no longer consistently deliver. For a charter group prepared to run north of the established circuit, the Mergui represents the Andaman equivalent of finding a surf break with no one on it.
Note that Mergui itineraries require advance permit arrangements through the relevant Myanmar authorities. The process adds planning lead time and cost but is manageable for a properly organised private charter. Discuss the current permit requirements with your charter broker well in advance of departure.
Sailing the Andaman Sea
Between the dive sites, the Andaman sailing is exceptional - and that matters for any charter where not every guest is a committed diver, or where the group wants more than a dive platform with beds.
Phang Nga Bay, north of Phuket, is the famous one: 400 square kilometres of sheltered water studded with vertical limestone karsts, accessible sea caves, and tidal hongs - hollow islands whose interiors become enclosed lagoons at the right tide. The bay itself is calm enough that sailing it is genuinely pleasant rather than simply a transit, and the anchorages around Ko Phi Phi, Ko Yao Noi, and Ko Hong are among the most photogenic in Southeast Asia. Ko Phi Phi Lee’s Loh Samah Bay - the curved lagoon where The Beach was filmed - is a proper overnight anchorage in settled conditions, and Ko Yao Noi has beach restaurants and local life that the Phuket development corridor has not yet absorbed.
The run south from Phuket to the Koh Lanta chain and beyond to Koh Ngai and the Trang Islands passes through progressively quieter water and fewer tourist boats. By the time a charter reaches Koh Muk - an island on the Trang coast with a tidal cave called the Emerald Cave that leads to a hidden inland beach - the boat traffic has thinned to almost nothing.
For a two-week charter based out of Phuket, a reasonable structure is: three or four days south on the Phang Nga / Phi Phi / Lanta sailing circuit, with day dive trips to the reef sites accessible from day-trip boats, then a five-day run north to the Similan Islands for the liveaboard-style diving, then the return via the Surin Islands and a night on Richelieu Rock if time allows. Groups with serious pelagic ambitions and more time should consider the extension north into the Mergui Archipelago: it requires more planning but delivers diving and exploration that the established circuits simply cannot match.
When to Go
The dive season is November to April, driven by the northeast monsoon that keeps the Andaman Sea calm on the west coast and holds visibility at 20 to 30 metres. The optimum window for both diving and sailing is December through March: northeast monsoon winds are consistent but manageable, seas are flat in the protected channels and moderate in the open water approaches to the northern islands, and water temperatures sit comfortably at 28 to 30°C.
February and March add the best whale shark probability at Richelieu Rock and Hin Daeng, which makes them the months to target for any itinerary with a pelagic focus. April marks the start of the season’s end: conditions remain generally good but begin to deteriorate as the southwest monsoon starts to build, and national park operators begin closing access to the Similan Islands from 15 May.
The Mergui permits and the associated planning logistics make the Mergui extension more practical as a December to February undertaking rather than a late-season addition.
If you are building an Andaman Sea charter - whether the focus is the established circuit or a push north into Burmese waters - talk to our team. The Andaman Sea has enough to fill several trips. The question is what matters most.
The Mu Ko Similan National Marine Park is closed from approximately 16 May to 14 October each year - exact dates vary and should be confirmed before planning. Permits to enter Myanmar waters for the Mergui Archipelago require advance arrangement through approved operators. Entry requirements and permit processes are subject to change; your charter broker should verify current requirements before departure.