The Similan islands are in a National park approx. 60kms offshore and have probably the best snorkelling in Thailand. I went there with a 3 day/2 night liveaboard trip from Poseidon Bungalows, Khao Lak.
Recommended !! This was the third trip I have done with them, over about 12 years, but the first with a decent camera underwater. The Canon G7X in a WP DC54 housing is a great buy !