Koh Samui has some wonderful site in the Antghong Marine Park. Sail Rock is also a great site wish is best taken from Koh Samui although is available from Koh Tao also but more expensive and takes so long.
I live and worked as an Instructor on Koh Tao. Two excellant sights to visit would be Chumphon Pinnacle and South West Pinnacle. Both are deep with some very interesting marine life and very large shoals of very large fish. Whale sharks are sighted on both these sights. I spent a number of dives with a mother and baby on the 30 March and first week of April last year on Chumphon Pinnacle.
Koh Tao you will also find less expensive. Out of the two it is the divers island.
The southern side of the island is far less "busy" than the northern side.
WHite Rock is an great night dive although a very busy site just after sunset.
Watch out for Trevor the Trigger Fish, he removed a big chunk of my buddies ear once.
AM sure you will enjoy yourself.
There is a dive company that also operates out of Chumphon. I did not get the time,
but I have heard a number of people say there are some very quiet and spectacular sites which the mainland operators use. There are many Islands just near Chumphon which is where you will catch a ferry from if you are not flying.
