Oh yes, just surfed the Internet a bit, and it turns out that Dophin Divers on Koh Chang doesnt speak about Dolphins (as the name suggests), but do tell that with some luck (probably a whole lot) Whale Sharks can be spottet in April/May…  that is gonna be interesting!
But Reef Sharks and some Porpoise, a subspecies of the dolphin, should be around… says Dolphin Divers..  Koh Chang seems like a great choice…  will tell a lot more when I have been there…  😉