A friend of mine made a bunch of films about his cenote diving. This one give a pretty good idea of what it's like: https://www.youtube.com/watch?v=99z8JgxdDmc
He also has some amazing IMAX footage of cenotes, among other caves, in his film Ancient Caves, which is still playing on a few IMAX screens, though it's run is mostly over. https://www.youtube.com/watch?v=MSZL9YbXDGs