My introduction to diving in Thailand was a live abord cruise on a catamaran from Phuket up to Similan, Surin and the Burma Banks. An experience I have never forgotten.
Similan is an archipelago of nine different islands. One of the best dive sites to see manta rays and if you are lucky whale sharks. Best done on a live aboard to dive sites like Richelieu Rock, Elephant Head, and Beacon Point.