Cenotes

Since this portion of the Yucatan lies on a porous limestone base, all the rivers are below ground.  At some spots you can get down to the water, which is very clear (and cool!).

Grande Cenote

That only LOOKS very shallow--at this point, the water's probably 10 feet deep.

Cenote Dos Ojos

This was an impressive Cenote, worth the entrance fee.  You could swim around the semi-circular area, with the only light coming from this slit that runs around the inside.  It's also WELL WORTH using some snorkeling gear.  The floor starts off fairly shallow towards the inside, but drops off abruptly on the other side, leading down to all kinds of underwater caves.  Part of an Imax movie was filmed in the underwater caves here.