Do not miss Chankanaab Park in Cozumel-great snorkeling, botanical and archeological park. Has restaurants, bars, showers, lockers and changing rooms.
Costa Maya appears to have alot to do-just put Costa Maya in a search engine-there is a resort there that will arrange tours-also snorkeling trips.
In Grand Cayman we did the Island Tour with Stingray City through the cruise company-we only had 5 hours there. Second time we snorkeled at Eden Roc-a five minute walk from the tender terminal-to the right.
In Jamaica we were at Montego Bay-we did a ship sponsored tour in the AM-rafting on a river-went to Margaritaville in the PM. We are from NYC and the people there were aggressive-I would do a ship sponsored tour there.