South of Cancun in Quintana Roo, the snorkeling at Puerto Morelos offers the beach-accessed Acamaya reef which is part of the Great Mesoamerican Reef. The off-shore reef is protected as the Puerto Morelos National Marine Park.
To reach the snorkeling in Puerto Morelos by air, you will need to fly into Cancun International Airport (Aeropuerto de Cancun - CUN). Many of the high-end beach resorts have an airport shuttle van, otherwise it is recommended to take a taxi. Renting a car to drive around can be hazardous do to the potential for unwarranted police stops resulting in required immediate cash payments. To reach the Puerto Morelos snorkeling from Cancun if you do plan to drive, head south on Mexican Highway 307 for about 20km to this small fishing village.
Note: while snorkeling here, keep in mind that from November through March you might encounter bull sharks right off the beach.
Overall, Puerto Morelos is the 7th most popular snorkel dive spot of all 40 snorkeling dives in Mexico.