I'm pretty sure it was Cenote Azul we went to last year, right beside the highway to Tulum.
It was very nice, locals swimming, pretty fish, cool water, lots of rocks to sit on (sharp on the bare feet!).
Wherever it was, there is a funny (?) story.
My daughter Laura, a tall 17-year-old, was on her way up to the concrete outhouses/changerooms perched up on a rock. Watching her footing and dazzled by the heat, she bumped into a low-ish horizontal tree branch. She hurt her head, and the operators got cold cloths and fussed over her, which was nice. Within 5 minutes, the fellow had his machete out and whacked the offending limb right off! (I have it on video!). So, let it be a lesson to trees: don't mess with Laura! (This should come in handy as she trains to be a bush pilot.)