Hey Kaniska,
We've been to Kep, which is a small fisherstown in Cambodia. Great krab btw. this is quite touristy but not nearly as touristy as Sihanoukville. We disliked Sihanoukville, we thought it was touristy, dirty and had a lot of old white men walking around with young asian company (if you know what I mean). We left immediately! A friend of mine went in the low season and throught it was fantastic though, and had a great time there.
At Kep you can do a boatttrip to Rabbit island, which is a very small island with white beach. You can stay there for a couple of nights! very serene and awesome. White beach, good swimming, have not snorkled so wouldn't know.
In Thailand I loved Ko Chang. One side is touristy and has some nightlife, the other side is totally serene and beautifull, rent a Scooter and explore!