Chankanaab — the name means "small sea" in Yucatec Maya — is a saltwater lagoon connected to the ocean by an underground channel, set in a botanical garden of palms, orchids, and reproduced Maya buildings. It was the island's first marine park and remains one of its most-visited day attractions.
Snorkelling from the beach is good and easy: brain coral, sea fans, and shoals of sergeant majors crowd the shallows just a short swim from shore. Inland, the lagoon is closed to swimming to protect the resident wildlife but you can walk a path round it. Expect crowds when cruise ships are in. Entry costs around US$25; bring your own snorkel gear to avoid rental fees.