For all its terrestrial wonders, the Yucatán Peninsula’s underwater treasures are no less compelling, including the world’s longest underground river system, the second-longest coral reef, and the Northern Hemisphere’s largest coral atoll.
Isla Cozumel , fringed by pristine coral reefs and remarkably clear water, is one of the world’s top diving and snorkeling destinations, while Isla Holbox  is a favorite feeding ground of manta rays and huge (but harmless) whale sharks.
But you need not leave the mainland to appreciate the Yucatán’s underwater realm—eerily beautiful cenotes and healthy coral reefs all along the Caribbean coast offer fantastic views and unforgettable experiences and can be enjoyed either on an organized tour or on your own.
Fly into Cancún  and head to Playa del Carmen , where the white-sand beach will be a welcome sight after hours on a plane. If you don’t have your own snorkeling equipment, arrange an extended rental at any of the dive shops in town. You’ll want to book a car rental too. Have dinner along Quinta Avenida.
Spend the day snorkeling and/or diving along mainland Riviera Maya . There’s great snorkeling on the reefs at Puerto Morelos  and Akumal . Akumal also has a fascinating lagoon, Laguna Yal-Ku , which is teeming with underwater life and features artful bronze statues along the shore. Dive shops in Playa del Carmen , Puerto Morelos , Akumal , and Tulum  all offer reef and cavern dives too.
Go cenote-hopping. Great options for snorkelers and divers are Siete Bocas and Lucerno Verde on the Ruta de los Cenotes , just south of Puerto Morelos; Jardín del Edén and Cenote Cristalino , both on Highway 307 across from Xpu-Há; and the string of cenotes  just west of Tulum on the road to Cobá, including Car Wash and Gran Cenote. For something truly spectacular, sign up for a guided snorkel or dive trip in the cenote parks Dos Ojos or Hidden Worlds .
Take the ferry to Isla Cozumel . Arrange a snorkeling trip  or a set of dives  for the next two days. After lunch, do some shopping  or head to the beach —either a beach club south of town or on Cozumel’s less-developed eastern shore.
Go on a snorkel or dive trip. Ask about taking a break (or your surface interval) at Playa Palancar , a gorgeous west-side beach with a low-key beach club.
Go on another snorkel or dive trip in the morning; Parque Nacional Chankanaab  and Faro Celarain Eco Park  (Punta Sur) both offer good snorkeling opportunities. In the evening, take the ferry back to Playa del Carmen and check out the nightlife  on Quinta Avenida.
Head south to Mahahual  or Xcalak , two small towns near the Belize border with fantastic snorkeling and diving right from the shore. Xcalak is the best jumping-off point for Banco Chinchorro , a spectacular coral atoll two hours away by boat. In the evening, take a stroll down the Malecón in Mahahual, or if you’re staying in Xcalak, go night snorkeling from shore (don’t forget an underwater flashlight!).
Spend this day snorkeling from the shore or taking a dive trip, either on the reef or at Chinchorro. Bear in mind that you should not dive within 24 hours of your flight home.