The Similan Islands are widely considered Thailand's best diving — clear water, manta ray and whale shark encounters, and pristine reefs in a remote marine national park.
The park closes May 16 to October 14 each year. This isn't optional weather closure — it's enforced by Thai marine authorities. Permits and entry are not sold during closure.
Liveaboard trips (3–5 days, ฿18,000–฿35,000) are the most popular way to dive Similan. Day trips from Khao Lak run during the open season but require very early departures (~7am).
Best months for visibility and manta sightings: February–April. December–January is peak season with highest prices and crowds.
Phuket is the closest international airport. Most operators include hotel pickup from Phuket or Khao Lak. Combine with Surin Islands for whale shark probability boost.