Had an excellent time on the Fury Catamaran Deluxe Sail, Snorkel, and Beach Party (booked through Carnival)! Hands down the highlight of our trip. HIGHLY recommend them. The snorkeling was kind of blah - not many fish to be seen - but the overall experience was amazing.
Cozumel shopping is always great, too. This was my third time to the island and I would happily return.