The Yucatan Penisula doesn't have rivers, instead it has cenotes, which are limetone wells. Most are underground like the two Rosa and I visited near Valladolid. The roofs of others have fallen in, making circular lakes. They used to sacrifice people in the one at Chichen Itza, the walls are so steep if you were thrown in you'd never get out.
Swimming in the cenotes can be pretty spooky. There are ones where you can dive, I'm not entirely disappointed I never made it to any of them... There is no way of knowing how deep the water is, but it's cold after the heat of the day above ground, and there are black catfish, and stalagmites, and bats....you're almost wondering where gollum is.