Map of Koh Tao, Thailand
If you want the best scuba diving in the Gulf of Thailand, head for Koh Tao. Get there from Chumphon or Koh Samui, but get there! Koh Tao has great beaches as well.
Best season for the Gulf of Thailand is June thru October. While November and December are high season elsewhere, sea conditions in the western Gulf of Thailand can get pretty rough.
NOTE: Many of the dive resorts on Koh Tao will not accept bookings from non-divers. Check before reserving. As to reputations of dive operators, research is recommended.