Mu Ko Similan National Park is an 11-island archipelago in the Andaman Sea off Phang Nga (Khao Lak), prized for granite-boulder swim-throughs on the west coast, white-sand reef slopes on the east, and the manta-and-whale-shark sites of Koh Bon and Koh Tachai to the north. The park is open only ~15 October–15 May and closes entirely to diving during the southwest monsoon.
Destination info
Conditions, highlights, and the resident marine life.
Conditions
Water and air temperature across the year.
WaterAirDryShoulderWet
Description
The Similan Islands lie roughly 65 km west of Khao Lak and about 100 km northwest of Phuket in Thailand's Andaman Sea, forming the core of the 140 km² Mu Ko Similan National Park. The chain is conventionally numbered 1–9 from south to north, with the satellite islands Koh Bon (#10) and Koh Tachai (#11) added to the park further north. The west sides of the islands are defined by enormous stacked granite boulders forming a labyrinth of swim-throughs, arches, caverns, and gullies (most famously Elephant Head Rock and Boulder City), while the sheltered east sides are gently sloping white-sand reefs and coral gardens that suit easier dives. Visibility is typically 20–30 m during the dry season and water temperatures run 26–30°C, though current-driven thermoclines can drop water 4–6°C colder at depth. The northern sites Koh Bon and Koh Tachai are the park's big-animal magnets: Koh Bon's southwest ridge is a reef-manta cleaning station, and Koh Tachai's current-swept granite plateau draws mantas, schooling pelagics, and occasional whale sharks, with the best big-fish window February–April. Critically, the entire park is seasonal: it is open to visitors and diving only from approximately 15 October to 15 May and is CLOSED for the southwest monsoon (roughly 16 May–14 October) to let the ecosystem recover and for visitor safety. Most diving is liveaboard-based from Khao Lak (typically 4-day/4-night trips), with speedboat day trips also running to the nearer islands during the open season.
Highlights
What makes this dive worth the trip.
Mu Ko Similan National Park is open to visitors only from approximately 15 October to 15 May each year and closes entirely during the southwest monsoon. For the 2025 season the park officially closed 16 May 2025 and reopened 15 October 2025; the current season is scheduled to close on 15 May 2026 and remain shut until around mid-October 2026. No diving, day trips, or overnight stays are permitted during the closure, which the park enforces for ecosystem recovery and visitor safety.
The west coasts of the islands are built from enormous stacked granite boulders that form a maze of swim-throughs, arches, caverns, and gullies, with Elephant Head Rock (Hin Pusar) and Boulder City the signature boulder dives, while the sheltered east sides are gentle white-sand reef slopes suited to easier diving.
Koh Bon and Koh Tachai, the park's northern satellite islands, are the big-animal highlights: Koh Bon's southwest ridge is a reef-manta cleaning station and Koh Tachai's current-swept granite plateau draws mantas, schooling barracuda and trevally, and occasional whale sharks, with the strongest big-fish chances February–April.
Marine life
46 species you’re likely to encounter on a dive here.
Dive sites
6 signature sites at this destination.
Elephant Head Rock (Hin Pusar)
An offshore cluster of three massive granite boulders roughly 1.5 km south of Similan Island #8, named for a rock formation resembling an elephant's head and trunk breaking the surface. Below, the boulders form a labyrinth of swim-throughs, sheer walls, arches, caverns, and gullies down to around 35–40 m on a sand base, draped with soft corals and sea fans. It is one of the Similans' premier deep boulder dives and is renowned for unpredictable 'spin-cycle' currents that can run in any direction and surge between the rocks, so it is best suited to experienced divers.
10–40 madvancedLiveaboardStrongVisibility 15–35 m
Christmas Point
A dramatic boulder site on the northwest of the Similan chain near Island #9, featuring house-sized granite formations with several swim-throughs and an arch around 24 m, sloping to a sandy bottom near 35 m. The deeper sandy zones are a good place to spot resting whitetip reef sharks and leopard sharks, while the boulders host soft-coral gardens and macro life including nudibranchs and ribbon eels. Conditions range from easy to more demanding depending on current.
5–35 mintermediateLiveaboardModerateVisibility 20–30 m
Koh Bon Ridge
Koh Bon is a limestone-and-granite island on the northwest edge of the park, about 20 km north of the main Similan chain. Its principal dive site is a submerged ridge on the southwest point that drops from roughly 15 m to over 35–45 m, functioning as a reef-manta cleaning station. Reef mantas are encountered regularly — often on the start of the incoming tide — and the site can produce sightings at almost any time during the season, with the strongest manta window typically February–May. The deeper Koh Bon Pinnacle nearby carries strong currents and is dived only in near-perfect conditions.
15–45 madvancedLiveaboardModerateVisibility 15–35 m
Koh Tachai Pinnacle (Twin Peaks)
The northernmost dive site in the park, on a granite plateau about 200 m off the southeast tip of Koh Tachai. The plateau rises from around 12 m and slopes down past 35 m, built from piled granite slabs and boulders with swim-throughs and channels, carpeted in soft corals, sea fans, and anemones. Strong currents sweep food up from the depths and concentrate large schools of barracuda, trevally, and tuna, plus visiting reef mantas at cleaning stations and occasional whale sharks. The current makes it an intermediate-to-advanced drift dive, best for big animals February–April.
12–40 madvancedLiveaboardStrongVisibility 15–40 m
Boulder City / Shark Fin Reef
A deep granite dive site at the far southern end of the Similan chain, off the southeast tip of Similan Island #3, where huge submerged boulders are scattered across a slope from about 5 m to 35 m. It is known for some of the best visibility in the area but can carry strong currents at times, making it a drift dive. Bumphead parrotfish, Napoleon wrasse, and leopard sharks resting on the sand are among the highlights, along with fan-coral gardens on the deeper boulders.
5–35 mintermediateLiveaboardModerateVisibility 20–35 m
East of Eden
A gentle east-coast reef in the central Similans (near Island #7) regarded as one of the park's most beautiful coral dives, with a healthy mix of hard and soft corals down a sloping reef and an isolated pinnacle. Calm conditions and a light current make it an easy, aquarium-like dive suited to all levels; green and hawksbill turtles are commonly seen on the shallow reef, and whitetip and leopard sharks rest on the morning sand. (A 2010 bleaching event damaged the famous fan-coral pinnacle; the wider reef remains a standout.)
5–30 mbeginnerLiveaboardLightVisibility 15–30 m
Where to dive & stay
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