QUOTE(jeremystravels @ Oct 23 2009, 07:12 AM)

What is an average price for a week long liveaboard there? I was looking at doing one. Also, if you were to pick the best two islands in Thailand to dive from, where would you go? I'm looking at going to all of the big ones, Ko Tao, Ko Samui, Koh Phi Phi, Phuket, etc.
If you are already certified, go to the Similan Islands. Ko Tao / Samui, as it has been said, use the same sites and these sites are for learners. As a result the coral has taken some beatings and there isn't an impressive amount of life down there. I haven't dived Phuket but have heard it is crowded due to the popularity of Phuket itself, and Phi Phi.
Liveaboards on the Similans are fantastic but yes I have never saw a week long either. Generally they are three or four days. The longer ones take you to better, more remote (but still popular) sites so I would definitely recommend doing the longest you can find.