All diving on Koh Chang is done off boats.  There's no beach diving off Koh Chang. (However, some dive schools prefer to complete any confined water training required on a dive course in shallow seas near the beach, rather than in a pool. It saves a few baht on pool fees.)

The map on the right pinpoints some of the main dive sites around Koh Chang.  There are quite a few more - these are simply the better known and those that you're more likely to visit. The rocky pinnacles and small islands around Koh Rang offer what most consider to be the best diving in the area.

Different dive schools prefer different dive sites, therefore prior to signing up for a dive make sure you check where you'll be going, your dive school will be able to give you details of the types of fish & marine life that have been recently seen at various dive sites and also the visibility.  You'll want to make sure that you'll be able to see something other than murky seabed before you hand over your cash.

Non-divers should note that the the snorkelling around Koh Rang is also particularly good with an abundance of fish to be seen.  If your budget doesn't stretch to a dive course or diving just isn't your thing then sign up for a snorkelling trip to Koh Rang, usually advertised as a '4 islands in a day' trip.

If you're a non swimmer, sign up for a snorkelling trip anyway.  Most Thais don't swim and that doesn't stop them from donning  a lifejacket and leaping in. Unless of course you possess a nagging urge to apply common sense to the idea of a couple of dozen non-swimmers going out to sea, with the one guy charged with watching over everyone but more often than not being easily distracted by a chick in a bikini on a nearby boat.

Which brings up the subject of insurance.  In brief, it's a good idea.