Mexico's underwater river (and friends).

The word "cenote" comes from a Mayan word meaning "well" and is used to describe sinkholes that have exposed rocky walls and contain groundwater. The water in these cenotes is often very clear since it comes from rain water slowly infiltrating through the ground. They are often the result of cave roofs collapsing to reveal extensive flooded cave systems beneath. These systems can have subterranean tunnels that reach all the way to the ocean. If such a cenote is a landlocked body of water whose tunnels lead to the ocean, it's what's known as an anchialine pool. In these pools, the pure rainwater floats atop the denser saline water from the sea. The sharp change in salt concentration over a small change in depth causes a blurry swirling effect due to the refraction between the different densities. Cool!
So one such body of water is the Cenote Angelita in Mexico. It's not an achialine pool, but goes straight down 200 feet. The first hundred are freshwater, and the last hundred are salt water--the extraordinary part is the layer of hydrogen sulfate that separates the two. Not only that, but there are trees at that depth as well, making it look like a most beautiful and eerie underwater river.
Quintana Roo, Mexico is home to the Ox Bel Ha ("Three Paths of Water") cave system, which is the longest explored underwater cave in the world, with 110 miles of underwater passages.
Other flooded cave systems include Dos Ojos, and the now combined Nohoch Nah Chich/Sac Actun cave system, both of which are also in Mexico near the Caribbean coast of the Yucatan Peninsula in Quintana Roo. Dos Ojos has no less than 25 sinkhole entrances and more than 20,000 feet of caves and tunnels. It also contains the deepest known cave passage in Quintana Roo, a cenote known as "The Pit" measuring at 396 feet.
These caves are super popular for diving, and of course, swimming!