For more information on the best time to go to the SImilan Islands click here.
Similan diving, including Koh Bon, Koh Tachai and Richelieu Rock is the best Thailand scuba diving on offer. The topography of the dive sites is varied, the water is crystal clear and sometimes the Similan Islands can be full of surprises.
Use these links to find exactly what you need to know about your Similan dive planning.
The diving season is from 15 October to 15 May!
From 16 May to 14 October the Similan National Marine Park is closed and entering within these months is illegal and also highly dangerous because of the prevailing weather conditions!
As the new year began, the Similan Islands saw steadily improving diving conditions. Early January brought some choppy sea conditions with persistent northerly winds, but by mid-month the seas had calmed significantly and visibility underwater improved. Water temperatures hovered between 27°C and 30°C (80°F to 86°F), providing comfortable conditions for divers. Marine life was abundant, with sightings of various reef species and occasional pelagic visitors.
February proved to be one of the best months of the season. The weather was reliably dry, and the sea conditions were excellent, with calm waters and visibility often reaching 30 to 40 metres. Currents were stronger at sites like Richelieu Rock and Koh Tachai Pinnacle, which helped bring in larger marine species and there were several giant oceanic manta ray sightings. Reef life was vibrant, and macro enthusiasts had plenty to enjoy as well.
Diving conditions remained favourable throughout March. Calm seas and high visibility continued, making it an ideal time for underwater photography and exploration. Nutrient-rich currents supported a wide array of marine life. Divers reported sightings of schooling fish, healthy coral reefs, and occasional pelagic species, making March another solid month for Similan liveaboard diving.
As the diving season approaches its end in May, conditions remain favourable. The sea stays calm, and visibility continues to be excellent, ranging between 20 to 40 meters. Water temperatures are consistently warm, providing comfortable diving experiences.
April also offers great opportunities to witness the Similans’ marine biodiversity. Divers can encounter a wide variety of reef fish, macro critters, and occasionally larger pelagics. It’s a rewarding time to explore the area before the seasonal closure begins in May.
In recent years, sightings of manta rays in the Similan Islands had almost completely stopped. However, the 2024 – 2025 season has seen a surprising and welcome increase in manta ray encounters—including at Richelieu Rock, a site traditionally known for whale sharks rather than mantas.
This resurgence may be linked to favourable ocean conditions and an increase in plankton levels, which are the main food source for manta rays. Whatever the cause, their return has added excitement to this season and renewed hope for more frequent sightings in the future.
If you are a novice diver, diving Similan Islands is often a wonderful experience.
Many of the sites offer easy diving conditions, crystal clear water and the good chance to see firm favourites such as turtles, anemone fish, moray eels, stingrays, pufferfish, lion fish, batfish and occasionally larger pelagic species such as barracuda and trevally.
If you include diving at Koh Bon which is often included in a weekly Similans schedule then there is a good chance to see Giant manta rays at certain times of the season.
The diversity of dive site types is a strong point of the Similan Islands. Gentle sloping coral reefs form on the east side of the islands, while rocky reefs dominate the west sides.
There are also a number of submerged pinnacles that form dramatic underwater seascapes and create adventurous dives through caves and swim throughs which adds appeal for more experienced divers.
Do not expect to be alone at the Similan Islands. With all guide books, internet sites and local operators selling the Similan Islands as one of Thailand’s top destinations you will find the islands and dive sites crowded. Unless your Similan Island trip includes divers, snorkelers or beach lovers in one group, avoid trips to the Similan Islands offering all activities from only the one large vessel, otherwise you will be snorkelling at dive spots or diving snorkelling spots whilst feeling the tremendous time pressure to complete your planned day! Thailand Dive and Sail recommend trips that separate their diving and snorkelling activities, with fast transfers to the Islands and also offering weekly schedules that include the better diving at Koh Bon, Koh Tachai and Richelieu Rock. Boats offering only a Similan Island and Koh Bon weekly schedule are not offering you the best of the Andaman Sea area.
Use these links to find out more about the breathtaking Similan Islands.
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