Renting a VW Safari (aka "The Thing" for $30 (insurance included)) and touring Cozumel on your own. The cars have Mexican "character" if you get the drift.
Head to Dzul Ha (FREE) for the best off shore snorkeling on the island, but you will need buy a few $1 beers (or if Ice cream is your poison) to keep the natives happy.
After a several hours of snorkeling, drive to the East side and enjoy the scenery and a choice of several stops for good food. On the way back to the rental (around 5:30 P.M.), put $5 of gas in the car and tour / shop downtown (there's parking on the streets), for about an hour or so. If you have late seating, you can get the car back by 6:00 - 6:30, plenty of time to walk to the ship and get ready for Mexicalli Night!