1) The attraction of Burma is that there are relatively few dive boats on the dive sites there. It is quite an extra expense to dive up there, particularly given that most boats doing Burma only dedicate a part of the trip to the area, spending the rest of the time in Thai waters.
2) What do you mean by "nicer"? A Peter Hughes-type vessel? Or something more workaday?
3) We don't see whale sharks with any regularity lately, so while you may be one of the lucky few who do see one, it's difficult to say when and where with any sort of confidence. Last month there were sightings near Phi Phi; they are sometimes seen in the Hin Daeng area; and they are also sometimes seen at Richelieu Rock in the Surins.