It's a beautiful island, there are many things to see in nature, some beaches are definitely too crowded though. It's also quite pricey but if you're patient and don't want to overpay you'll find reasonably priced restaurants and accomodation.
After spending some time under water we decided to stay on the surface for a while and tried stand up paddling. It's an interesting exercise, but a bit boring for me. If it's done at the sunset - it's totally different then. Beautiful thing.
Some places are a bit more expensive than others but usually it means there's more fun included, either better place or longer experience. Worth to raft in Krabi, it's really great.