If you're going to spend a whole month traveling around southern Thailand, you can pretty much dive everywhere there's diving on offer. There are islands in two main parts of the country--the Gulf of Thailand to the east and the Andaman Sea to the west. You could spend a week or so exploring the dive sites around Koh Tao and Koh Samui in the Gulf, and then three weeks or so in the Andaman. If I had that amount of time available for the Andaman here's what I'd do:
1) I'd get on a liveaboard to the Similans and Richelieu Rock.
2) I'd do a couple of daytrip excursions from Phuket to Shark Point and Phi Phi to round out the first week
3) I'd take the Tigerline Ferry south, stopping off at Koh Lanta for a few days to dive at Hin Daeng/Hin Muang and Koh Haa, then hop back on the ferry to the next island, and so on.
4) The little islands on the ferry line don't offer spectacular diving, but they are relatively undeveloped and unspoiled, so you can chill for a couple of days at each one, do a couple of dives, relax on the beach, and then keep going. At this point maybe two weeks of the three are used up.
5) I'd get down to Koh Lipe and dive for several days there at Eight Mile and Stonehenge among other places and enjoy the island vibe.