Part of the Cozumel Reefs National Park (or Parque Nacional Arrecifes de Cozumel) Faro de Punta Celerain, also known as Punta Sur, Ecological Park offers some of the best diving and snorkeling around Cozumel. If you want to dive, go through one of the island's many dive operators. If you'd just like to snorkel, however, you can rent equipment and guides right here.
In addition to the undersea attractions, Punta Sur has broad, beautiful beaches (the reef is well offshore, so you can splash around safely), great seafood, and shady hammocks. If you're up for a some terrestrial exploration, you could climb the Faro de Punta Celerain (Celerain Point Lighthouse), with great views, or visit the tiny Mayan shrine to Ixcel, the fertility goddess, known as Tumba de Caracol.
Punta Sur also has interesting wetlands, a magnet for migratory birds in April and May, and home to lots of crocodiles year-round.
There is no public transportation to Faro de Punta Celerain (Punta Sur) Ecological Park, located 30 kilometers (18 miles) south of San Miguel, at the southernmost tip of the island. You'll either hire a cab or rent a car. Dive operators include transportation, of course, but be aware that this area, due to its depth, cavern system (including famed Devil's Throat) and currents is for experienced divers only.
While the park entry fee includes beach access, note that the reef is well offshore, and some visitors may not feel comfortable swimming all the way out without renting either a life jacket or kayak guide - the latter will also show you around the damaged coral. Other attractions, including the lighthouse and wetlands, also charge small extra fees.