All the atolls have something good to offer. I love snorkling at Turneffe..but you have to stay a week or pay big bucks for transport. You can go to Dangriga and catch daily boats to Tobaco Caye. If you are already going to be on Ambergris I would suggest going up to the very north of the island where the reef meets land and check out Tranquility Bay..it is just too kool for snorkling, and like you are on another island in another place....