I just came back from Krabi 6 months ago - to be honest it is hard to give any lens advice because unless you are shooting something in particular it is like any other beach town. I am not saying it is ugly by any means but just bring what you would normally bring on any other vacation.
A wide, a telephoto, and your favorite day lens would probably suffice.
Honestly I always take more than I need and end up kind of regretting carrying so much gear.
And if you end up taking one of the boat rides out to the islands its not like you really want to switch lenses with all the salt water being thrown around anyway. My 24-105 broke due to the jarring of the boat ride (luckily I have insurance).
Thanks! I guess I'll just be carrying the G11 with underwater casing for the boat ride. It's not a risk I think I can take. We're planning on island hopping and snorkeling. I guess I'll have to restrict my DSLR use on the beach near the resort. Is it advisable to go to the shell cemetery? Is it worth to bring a macro there? How about the other tourist places such as the Tiger Cave Temple? Do you have any experience going through those?