you must go to Chankannab Park the place to snorkel baby -- whip out to the taxi stand and head for this beautiful park. Admission is $10/person but it's worth it--a lovely way to spend the day. Restaurants, snorkel rentals, gardens and a little musuem
Get to Chankannab National Park as early as possible.
To see a photo I took there http://www.cancunsteve.com/cozumel4.htm