I just want to re-iterate what 2toDive just said. The FL Keys are not a good snorkeling venue. The only good snorkeling is 7 miles offshore on the reef, which limits you to snorkeling on tour boats. Also, winter in the Keys can be windy and cool. When strong cold fronts come through, it may be too windy for the boats to go out. If snorkeling is your main interest for a vacation, I would consider going elsewhere.