On Cozumel, try Chankanaab Park. It's a short cab ride from downtown or the International Pier and the admission fee is low ($5 US I think). The restaurants at the park serve surprisingly good nachos and margaritas and it makes for a nice day though the snorkeling is not necessarily the best on the island. They have plenty of gear available for rent. Dzul Ha is another good spot. Most cab drivers will know it and there is a small beach bar nearby for refreshments. I think they also have a limited amount of snorkel gear for rent.
If you have a long day in port, you could also try to visit Xel-Ha on the mainland. Fabulous sea life to view, but not a lot of coral. Yal-Ku Lagoon in Akumal is another great spot, but may be too isolated for an easy day trip.