That is where it all began (unknowingly) for me...
I dove at every stop we made along the unbeaten path. The best diving I experienced was at Koh Lipe in the Taratao National Marine Park (West Coast, southern end of Thailand) right a the border of Malaysia almost (where we also did a visa run).
Anywhere I dove off the shore of an island, brought interesting things such as trumpet fish, octopus, rays, eels, huge tiger clams(name?), corals, etc... Not always abundant and some reefs previously destroyed by hurricanes/storms. But there was always something interesting.
That being said, we did not target any specific area for its diving. But definately once we got out to Koh Lipe and then took a snorkelling boat tour around the area, we were greeted with spectactular sites of corals and its denizens. Even snorkelling on the shores of Koh Lipe we had lots of corals and formations to explore.
However, along the way you will meet lots of people in guest houses, and many will have experiences to share, just start asking about where they have been. Your bound to have a few that are divers. It all opens up from there...