The Andaman
The Similans and Koh Bon are the best sites in the country: granite boulders, visibility to 30 metres, manta rays at Koh Bon from February. The park closes May to October. Liveaboards from Khao Lak are the efficient way to do it.
Day trips from Phuket and Khao Lak reach Koh Haa and the King Cruiser, which are good rather than exceptional.
The Gulf
Koh Tao is the training island — thousands of certifications a year, cheap courses, and decent reef at Chumphon Pinnacle and Southwest Pinnacle. Visibility is best March to September.
Sail Rock, between Tao and Phangan, is the best single dive in the Gulf.
Choosing an operator
Ask for the certification level of your instructor, group size (four divers maximum per guide is a good rule) and whether insurance is included. Cheap courses with eight students per instructor are cheap for a reason.
