Koh Tachai one small of Similan Islands , there is a new island in the Andaman coast the best for snorkeling and scuba diving also white sand and clear water.
Koh Tachai only run for a day-trip, no accommodation around there, that why this place it's worth to go (for diver).
Most divers who would like to see beautiful corals,Manta Rays, Leopard Sharks and if get lucky you might see Whale Shark!
If any diver get a chance to come to Thailand , you might like to visit this amazing islands.

more detail : you can stay at Khao Lak and hire speedboat (buy package tour) to Tachai cost about 3,200 - 3,500 THB depends with tour program.