We are going to Chankanaab this year, Haven't been there since Wilma, so it will be interesting to see how it is. On Roatan we are going to CoCoView Resort to snorkel. On Grand Cayman there is a nice place to snorkel within walking distance of the tender dock. It is Abanks'.
It is free and you can rent chairs, lockers, and equipment. Lots of fish. They also serve some pretty tasty conch fritters there.