Old story. Best place for diving in Thailand is Similan Islands. It's about 100km from Phuket to north west on Andaman Sea. Dive trips to Similan Islands are usually liveaboard. Typical is five nights on the boat and four diving days. Four dives a day so 16 dives on the trip. I have been on this kind of diving safari twice. Feb 2006 and Feb 2007.
Similan Islands is not good only for diving. It's also superp for snorkeling. One of the most beautiful place there is Donald Duck Bay. There you can climb up to the hill where opens great view to the bay. I visited there in 2006.
After view point we had still time to wait for the next dive. Our boat was on the bay and we saw turtle very near of our boat. People threw bananas to the turtle and turtle ate it. Some of us jumped to the water snorkeling with the turtle. Of course me too. Turtle wasn't shy at all. It just swam around us.
It came many times very close. I didn't need to use zoom at all.
There was also smaller turtle, but it didn't come to close.
Great snorkeling session!
Mika