Chocolate (on front street) used to take folks to Goff's Caye as well - I remember some good trips with him, while on the island stop by and see who he recommends as well. The Caye has suffered a lot of abuse since the cruise ships started large scale snorkeling there, but the elkhorn coral is beautiful. The deeper water snorkeling is still good as most don't go out very far. I am pretty sure that the local tour group(s) recommended here can fill you in on which days this winter the cruise ships are in harbor as the schedules are pretty much set up in advance (weather being possibly the only limiting factor.)