You should probably post in the Asia forum to get more answers to your questions. There are actually a good number of posts in that section of SB on Thailand hotels and liveaboards.
This section is for organizers/sellers and participants/buyers of group trips.
I'll be going to Thailand this November and doing day trips with
West Coast Divers. They have liveaboard that was recommended to me by a friend, but I was unable to fit it in my schedule.
If you want to hang out on the beach, on a secluded resort, and dive around Phi Phi Island, then I would recommend
Phi Phi Island Village Resort. In October, they usually run a special. So if you do a liveaboard for Similan, I would do Phi Phi Island Village Resort to dive Phi Phi Island.
As for sight seeing, while you're in Bangkok you have to see The Grand Palace, Wat Phra Kaeo, and Wat Po. Those can all be done in one day, if you start in the morning. There is a dress code so make sure you have proper attire. When you reach Wat Po, go get a massage at the traditional thai massage school (the original in Thailand). If you're in Bangkok, then you should venture out to Chatuchak weekend market. Very hot but a great please to wander around and get lost.