Akumal, just 5 kilometres south of Puerto Aventuras on Highway 307, has one of the Riviera Maya's most reliable sea turtle snorkel sites. The crescent bay is sheltered, and green sea turtles come here year-round to graze on seagrass in the relatively shallow water. You can snorkel out from the beach and find them without a guide, though guided tours are available at the entrance and help ensure you keep a respectful distance.
The reef just beyond the bay is also good for snorkelling — parrotfish, angelfish, and spotted eagle rays are regular sightings. The beach itself is pleasant and the water is generally calmer than the open coast. Arrive before 9am to beat the tour-group rush; by mid-morning the bay can be crowded. Gear rental is available on the beach.