There are big, well known cenotes, and small "unknown" cenotes as well. They are everywhere. Ask the local children, when you travel, where their favorite "swimming hole" is? There are several small cenotes right in Playa del Carmen, and some larger ones suitable for swimming within just a few miles.

A particularly nice one is Cenote Azul, about 4 kilometers from Puerto Aventuras. Fantastic fresh water swimming of course, but it's specially a delight to cave divers because this is a principal entry point to visit many other cenotes.

BIG caution to divers - you must be cave-dive certified to dive these underground rivers. It's easy - and deadly - to get lost. There are experienced professional dive-masters in Playa who are familiar with these cenotes and rivers and have the specialized equipment (lights, rope etc.) needed to dive them safely. Take advantage of their knowledge and services.

Not only do they provide fresh drinking water for the people, but also for the indigenous wildlife... which is where easier hunting comes in. The vast majority of the Yucatan is parched for most of the year, so it can't be overstated how important the cenotes were to the original inhabitants. It is worth nothing that all the (well known and secret) sites associated with the Maya are located where cenotes exist. It seems the bigger the cenote the larger the site. No coincidence. Because these cenotes provided such an important and basic natural resource, it is only natural that the ancient Maya included these cenotes in their religious rituals.  For diving info: Dos Ojos Dive Center