Grand Cayman has the feed the stingrays boat trip (awesome) and is great for snorkeling. Cozumel also has great snorkeling but try renting a car and tour the island. The eastern side has beautiful beaches and many beach bars. I think the problem with all these destinations will be the crowds. Most all cruise ships will be going to the western Caribbean in the winter. Good luck, if it were me, I would try to cancel but I don't think the cruise lines will let you. They will find a substitute itinerary.