Tulum, Mexico, is a magical place that has never yet seemed to be to crowded or overrun. It is the smaller sister of Playa Del Carmen on the Yucatan Peninsula. The whole area is made of limestone and lowland jungle with amazing, crystal-clear water and beaches!
Underground river systems and caves/caverns are all over the area, which can be snorkeled and dived with the right gear and instruction. Just outside the north end of Tulum, off the main road, is Cenote Calavera... A short hike from road through forest will bring you to a surreal opening in the ground, non-descript and desolate except for a ladder that runs down to the water about 15 feet. This is an opening to the river system, and one can dive into the river into a series of caves and tunnels. With correct gear and guides, it is a wonderful experience to cavern dive, and find yourself in 80 ft rooms with water so clear, it seems you are floating in space! But you can also experience the cenotes from the surface with snorkel gear: Jump in and snorkel around in the fresh water, and you can see down to the cenote floor, and further into the cave system, because the water is so clear and crisp... So great to beat the Yucatan heat, and find respite in one of the cenotes as well.. Check out Cenote Divers in Tulum for info and gear. They are great guys from Norway who have been there for over a decade guiding and exploring through the Mexican cenotes.