There are a range of good dive spots around the lagoon and the islands that fringe it. (Dive-The-World.com)
I often read that the Manta Ray Bommie is amongst the ten best dive spots in Australia. (myfavouriteescapes.com)
The diving may not be quite as varied as in Phuket and the Similan Islands, but there are some very good dive spots around Samui and Koh Tao which can be combined with a stay at one of Samui's excellent resorts. (divetheworldthailand.com)