Coral island, sakai Beach, bamboo island. phi Phi island and James Bond island are bit crowded. you can check out our thailand itinerary on out tripoto profile for more details.
Koh Lanta - try it, and you'll definitely love it! It's relatively unexplored, and has some of the most beautiful views on deserted beaches, extremely jovial people and cute little cafes. The place is a like a throwback to old small towns, where the population is less and the love is more. You can go for snorkeling packages, enjoying the best of both the activities of the sea and the peace of the beach.
Go for Koh Samui, Phi Phi Island, or even you can visit small islands from Krabi which are not too much crowded but you won't be able to find a stay there.