I assume you are doing the Western Carribean 7 day cruise through Montego Bay, Grand Cayman, and Cozumel?
If so, the one thing you have to do is swim with the Stingrays in Cayman. It is an absolutely AWESOME experience. Nothing like it in the world. Cayman is a wonderful place. Great water, terrific climate. You'll love it. If you have dive equipment, use it there.
At Cozumel we went to Playa Del Sol and had a good time. I hear the National Park is pretty. We did the Mayan Ruins at Tulum a year ago and it was a LONG tour and if you don't like history you won't like it. But the BEACH there is wonderful.
Jamaica was a good place to snorkel. I would look for snorkel/diving tours there if it were me.
Good luck and have fun.