I just finished my DM course in Koh Lanta, and I had a great time. It being low season, the bottleneck was frequently not enough customers to take the boat out, and a couple of weeks of bad weather. I was there from the 2nd week of june till end of August, in which time i did my advance, EFR/First aid, Rescue, nitrox, deep dive and DM. From Koh Lanta, we dived Koh Ha, Phi Phi, the Bida's and one trip to Hin Daeng/Muang. Even in low season, it was great diving, with visibility at its worse around 15m and plenty of days of 30m vis. Also, lots of professional, friendly dive shops (I trained at Lanta Diver and I cannot say enough good things about them).