Between roughly June and September the largest fish in the sea gathers north of Isla Mujeres in numbers found almost nowhere else — hundreds of whale sharks feeding at the surface on tuna spawn. You snorkel with them, never dive; boats carry a licensed guide, two swimmers enter at a time, and touching is prohibited and enforced. It is the island's defining trip and it is worth planning a visit around.
Isla Contoy, thirty kilometres further north, is an uninhabited national park and one of the most important seabird colonies in the Mexican Caribbean. Visitor numbers are capped by permit and you can only go with a licensed boat, which is the whole point.
Closer in, the Garrafón reef at the southern tip and the MUSA underwater sculpture museum — more than five hundred concrete figures sunk between here and Cancún to grow coral and take pressure off the natural reef — are shallow enough to snorkel and interesting enough to dive. Sport fishing off the island runs year-round, and the sunset sail is the low-effort evening.