Cancún sits at the northern tip of the Mesoamerican Reef, the second-largest barrier reef system in the world, which makes solid snorkeling and diving accessible without a long boat ride.
MUSA — the underwater museum
The Museo Subacuático de Arte is a genuinely unusual attraction: several hundred life-size sculptures placed on the seabed off Isla Mujeres and Punta Nizuc, designed in part to draw divers and snorkelers away from the natural reef and reduce pressure on it. Visibility and the exact sculptures you'll see depend on which gallery your tour visits.
Reef snorkeling basics
Look for operators who brief you on not touching or standing on coral, and who use reef-safe sunscreen policies — the reef here has faced real bleaching and damage pressure, and small individual choices add up across the volume of visitors Cancún gets.
Diving
Certified dive shops around the Hotel Zone and Puerto Morelos run reef and cenote dives; cenote (cavern) diving requires separate certification from open-water reef diving, so check requirements before booking if that's the plan.