Cancún's own water is not its best feature — the hotel zone fronts open sea with a steep beach and strong currents, and the reef is thin. What Cancún is good for is reaching everything else.
The MUSA underwater museum is the exception close to hand: more than five hundred concrete figures sunk between Punta Nizuc and Isla Mujeres, now growing coral, shallow enough to snorkel and odd enough to be worth a dive. It was built deliberately to take visitor pressure off the natural reef, and it works.
Whale sharks run from Puerto Juárez and Isla Mujeres between May and September. Isla Mujeres itself is twenty minutes by ferry. Cenotes start forty minutes inland and the Riviera Maya shops will collect from Cancún hotels. Chichén Itzá, Cobá and Tulum are all day trips, as are Isla Contoy and Holbox.
Sport fishing out of the marinas — sailfish, marlin, dorado — is the other thing the city does properly, and the boats are serious.