Hi there,
Have dived Similans, they are amazing and have managed to cope quite well after the Sunami, only some minor damage to coral reefs. I went with a company called Pro-Similan Divers who do live-aboards at a very reasonable price. There season is mid Oct through to end of April with the best time being Dec-Mar, but it is always good. Large fish, leopard shark, turtles and Dec you get to see Manta ray - some strong currents so a lot of drift dives. There are other companies, but I found these guys who have been operating for years, know exactly where to go and avoid all the other larger dive boats.
I have dive shop in Koh Samui so often go diving there. If you are interested or want any further info you can contact me on our website.
As for camping, they did have a campsite in Khao Lak but not sure of the name, you could check out the lonelyplanet website...
Have a great time.
2Dive
info@a-seadivers.com