Chankanaab National Park
If diving or tours are not what you're looking for, there are still plenty of other things to do:
The park includes a beach, restaurant, bar, changing rooms, walking trails, and a lagoon with a large iguana population. Activities include snorkeling & diving, swimming with the dolphins, snuba, and Seatrek. $2 coupon and packages available at their website.
Paradise Beach
Free admission, beach chairs $2. Beautiful sandy beach, one of Cozumel´s largest swimming pools opening Dec.15th, 2010, a $12 FunPass is available for unlimited access to over 14 land&ocean activities, including snorkel equipment, kayaks, paddleboards, waterslides, water trampolines, bungee trampoline, coconut tree climb, a 14-ft. climbing iceberg, and more. Fabulous food and drinks served right on the beach or at the restaurant. Clean bathrooms, showers, change rooms and lockers. Beachwear boutique, parasailing, waverunners, massage, etc. also available. Located 9 miles south of San Miguel Downtown - Cozumel, 5 miles south of the International Cruise Ship Pier.
Deep Sea Fishing
(http://thisiscozumel.com/...) — cozumel has a wide range of days tours covering an enormous number of activities. many are designed primarily for cruise ship passengers that spend a day on the island, but are also suitable for hotel guests or people in rented accommodation.
Cozumel is rated as one of the top ten sport fishing "hotspots" in the world. Fishing is enhanced by the deep-water channel between the Yucatan Coast and the Cozumel Island. The channel "squeezes" the Gulf Stream as it passes by Cozumel on its journey northward. This "funnel" effect causes an upwelling and an abundance of fish. The nutrient rich Caribbean waters around Cozumel support a vast array of sport fish with exceptional fishing opportunities all year long.