I've just been in Koh Phangan, Thailand and was diving on "Sailrock". Around Koh Phangan itself are some cool dive sites too but for sure the best dive site in the Gulf of Thailand is Sailrock. Most of the diving companies from Koh Phangan schedule their dive trips either to Sailrock or to the neighbor island Koh Tao where there are a lot more dive sites around the island.
When you want to dive from Koh Phangan, I recommend to start with one of the diving companies in Chaloklum, in the north of the island. This avoids long boat trips to the dive site. The same for dive trips to Koh Tao, Chaloklum is the nearest place on Koh Tao and has a very good pier. There is a website about
Chaloklum Koh Phangan that has many pictures and also links to all diving schools in Chaloklum, if your interested.