It's very dive-centric, but one place that springs to mind is Scuba Club Cozumel, which offers unlimited shore diving on the reef just in front of the property. Better if you can spend some time under the water, but shore snorkeling here is certainly a possibility. They offer introductory scuba courses as well as more technical training if you're interested. More info at: http://bit.ly/hY5wud

You might also try the Barcelo Maya, a huge Riviera Maya all-inclusive with an excellent on-site dive center, Dressel Divers, that runs diving and snorkeling trips to the offshore Mesoamerican Barrier Reef right from the beach at the resort. More info at: http://bit.ly/evLgw8