Just to add on Lanta Diver, I spent the summer there doing my Divemaster course with them and have nothing but great things to say. They are incredibly well run, professional and friendly. Capt Singh is the most respected captain on the island, the instructors are thorough and patient, and the divemasters know the sites like the back of their hand (some have thousands of dives from Lanta). But there are also plenty of other shops, and they are respectful and cooperative with each other. Often times in low season, we'd share boats. So I can say from first hand experience that Pimalai divers, palm beach, scubafish, dive and relax and godive are all professional shops that I would dive with again. I'm sure there are others, but i've had personal experience with the above.