The waters around Isla Mujeres sit at the northern edge of the Mesoamerican Barrier Reef, the second-largest coral reef system in the world. The reef is accessible from multiple points on the island's western and southern shores, with visibility often exceeding 20 metres in the dry season. Nurse sharks and rays are common; whale sharks pass through the waters north of the island in large numbers from May to September.
A short boat trip from the island brings you to the MUSA — the Museo Subacuático de Arte, an underwater sculpture museum that has become one of the more extraordinary dive and snorkel sites in the Caribbean. More than 500 life-size human figures created by British sculptor Jason deCaires Taylor stand on the sandy seafloor in around 4 metres of water, slowly colonised by coral and sponges. Snorkellers can see them clearly from the surface; divers can move among them. The effect is eerie and beautiful in equal measure.
Several dive operators on the island run trips to MUSA, to the Garrafón reef, and to Manchones reef further south. The whale shark season (May–September) brings dedicated tour boats that operate under strict regulations — swimming with whale sharks is controlled to avoid harassment of the animals.