Cozumel is a drift-diving island. The reefs run along the sheltered western side, the current runs south to north, and the technique is to get in, stop swimming and steer. Palancar, Santa Rosa, Columbia and Punta Sur are the names, and the common feature is a wall: coral built up to the edge of a shelf that then falls away into blue.
Visibility is routinely thirty to forty metres, the water is warm enough year-round that the wetsuit is a formality, and the whole western shore has been a national marine park since 1996 — every operator needs a park permit and every diver pays the fee.
The island is also the easiest base for a mainland cenote day: the ferry to Playa del Carmen takes forty-five minutes and most shops sell the trip. Whale-shark snorkelling in June to September means a longer day up to Isla Mujeres.
Snorkellers do well here too. Dzul Ha and Money Bar are reef off a beach rather than reef off a boat, and El Cielo in the south is a shallow sand flat full of starfish reached by small boat.