Koh Khrok is a finger shaped islands that sits right along the tidal flow line and therefore is a great place for a drift dive.
The island has lots of sea anemones and their attendant clown fish.
In fact if you get close enough you can actually hear the clown fish snapping at you trying to convince you to go away!
The site has a maximum depth of around 11 meters, but is long enough that even with quite a reasonable drift a 45-50 minute dive can easily be done.