thailand is one of my favourite spots in the world!!!
there are so many different islands which offer lots of different holidays.
you said you like diving? ko tao would be the best diving island. ive oly been there once, in 2005.. and i didnt dive, but this enitre island is dedicated to diving. when i went there it was very relaxing, you could get a cheap or luxury type villas and the town is very chilled out. many cafes offering thai /western food and quiet markets for shopping. the only problem is getting there, not so straight forward. you would fly to ko samui, then get a 3 hour ferry.
ko samui is also great, surely there are diving spots there too. ko samui has a quiet side where you can relax by beach, go to fishing villages for fresh seafood, markets etc plus a few bars here and there.
ko phi phi and ko panghan are great, but are quit busy with bars and travelers. lots of fun, but not so chilled out.
my favorite island is ko samet, it has one main beach, very quiet, and along the beach is a row of thai restaurants, all set on the sand. this place has great luxury villas and its not touristy.. actually alot of the tourists i see are locals. theres also a few bars here too.
hope you have a great trip!!