Similan Islands & Richelieu Rock
The Similan Islands are nine granite humps in the Andaman Sea off Khao Lak, their western sides stacked with house-sized boulders and swim-throughs, their eastern sides draped in coral slopes. The national park opens only from about mid-October to mid-May. Liveaboards sweep north past the manta ridge at Koh Bon and Koh Tachai's dome to Richelieu Rock, the purple pinnacle that stands as Thailand's single most celebrated dive.
Why dive here
This is Thailand's blue-water showcase, and the geology alone justifies the boat ride: granite boulders the size of buildings lean against each other into mazes at Elephant Head Rock and Christmas Point, with sand channels, archways, and shafts of light that feel architectural. Northward, the sites grow toothier. Koh Bon's ridge is the country's most dependable manta station, Koh Tachai's submerged dome gathers trevally and barracuda in ripping current, and Richelieu Rock concentrates everything from whale sharks in season to tigertail seahorses and harlequin shrimp on one horseshoe of soft-coral-covered limestone. The closed season matters as much as the sites: five months of monsoon rest each year, park patrols, and mooring rules have kept the area in shape. Most divers go by liveaboard for two to four nights, though fast day boats from Khao Lak now reach even Richelieu. Currents reward experience; the payoff scales with it.
Month by month
Mu Ko Similan National Park closes annually for the southwest monsoon, opening from roughly 15 October to 15 May; within the season, December to April is primest, with whale-shark odds at Richelieu Rock highest from about February to April.
| Month | Water | Visibility | What it's like |
|---|---|---|---|
| January●●●●● | 27–29 °C / 81–84 °F | 15–30 m | High season; calm seas and the full route open. |
| February●●●●● | 27–29 °C / 81–84 °F | 15–35 m | Prime conditions; whale-shark window opens at Richelieu. |
| March●●●●● | 28–30 °C / 82–86 °F | 15–35 m | Warm water and the year's best big-fish odds. |
| April●●●●○ | 28–30 °C / 82–86 °F | 12–30 m | Plankton blooms feed mantas; visibility trades down. |
| May●●○○○ | 28–30 °C / 82–86 °F | 10–25 m | Season ends mid-month as the monsoon arrives. |
| June●○○○○ | — | — | Park closed for the southwest monsoon. |
| July●○○○○ | — | — | Closed; boats overhaul in port. |
| August●○○○○ | — | — | Closed season continues. |
| September●○○○○ | — | — | Monsoon closure; no access. |
| October●●●○○ | 28–29 °C / 82–84 °F | 10–25 m | Park reopens around mid-month; seas still settling. |
| November●●●●○ | 28–30 °C / 82–86 °F | 15–30 m | Early-season diving with building visibility. |
| December●●●●○ | 27–29 °C / 81–84 °F | 15–30 m | Holiday season; conditions firmly in place. |
Monthly figures confidence: Well documented. Months without a row have no verified data — we would rather show a gap than invent one.
The dive sites (6)
| Site | Type | Depth | Level | Access | |
|---|---|---|---|---|---|
| Christmas Point Archway sequences, ribbon eels and Napoleon wrasse, northern-Similan… | Reef | 8–35 m | Intermediate | Liveaboard | |
| Elephant Head Rock The archipelago's best swim-through maze, light-shafted granite rooms… | Reef | 8–40 m | Advanced | Liveaboard | |
| Koh Bon West Ridge Reef mantas at cleaning stations, a diveable ridge line in all currents… | Wall | 10–40 m | Intermediate | Liveaboard | |
| Koh Tachai Pinnacle Trevally tornadoes in ripping flow, manta and whale-shark transits… | Pinnacle | 12–36 m | Advanced | Liveaboard | |
| Richelieu Rock Purple soft-coral pinnacle, seasonal whale sharks, world-class macro on the… | Pinnacle | 2–35 m | Advanced | Liveaboard | |
| Shark Fin Reef Kilometer-scale granite spine, rays and garden eels on the sand… | Reef | 5–35 m | Intermediate | Liveaboard |
Marine life you can plan around
Blacktip Reef Shark
Blue-Spotted Ribbontail Ray
Broadclub Cuttlefish
Leopard (Zebra) Shark
Lionfish
Oriental Sweetlips
Peacock Mantis Shrimp
Titan Triggerfish
Leaf Scorpionfish
Reef mantas at Koh Bon and Koh Tachai, seasonal whale sharks at Richelieu Rock, leopard (zebra) sharks on the sand, schooling trevally, barracuda and snapper, banded sea kraits, and a strong macro bench of seahorses, ghost pipefish, and shrimp.
Getting there & staying
Getting there: Fly to Phuket, transfer about 1.5 hours north to Khao Lak, and board either a liveaboard or a speedboat from the Thap Lamu area; the islands lie roughly 60-70 km offshore.
Where to stay: Almost everyone sleeps on a boat or in Khao Lak; park accommodation ashore is minimal and diving from it impractical. Khao Lak offers everything from hostels to resorts along a long beach strip.
Liveaboards: Liveaboards from Khao Lak and Phuket run 2-6 night Similan-Koh Bon-Koh Tachai-Richelieu circuits all season and remain the best way to hit the northern sites at the right tide.
Costs: Day trips from Khao Lak with two dives commonly land in the US$130-190 range including park fees; liveaboards of 2-6 nights are the classic approach and usually cost less per dive.
Between dives: Khao Lak's beaches, waterfalls, and the tsunami memorials; within the park, snorkeling stops and a viewpoint scramble on island 8 during surface intervals.
Safety & practicalities
Hazards: Strong and sometimes unpredictable current at Koh Tachai, Richelieu, and the boulder points; surge in swim-throughs; long boat rides in open sea.
Carry a surface marker and listen to the current briefing; several marquee sites are negative-entry, hook-less drift affairs when it runs. Day-trip schedules are long; hydrate and mind repetitive-dive limits, with nitrox widely available.
Nearest recompression: Hyperbaric chambers operate in Phuket, a road-and-boat evacuation from the park. Availability changes; all emergencies through EMS and DAN.