Outside of Chichen Itza is a cenote called Xtoloc or 'Well of Xtoloc'. It's known as the 'Blue Cenote' by the travel guides etc.
A cenote is a sinkhole in the limestone, filled with freshwater. It was where the natives came to get water for thousands of years. They are scattered all over the Yucatan penisula. There are no rivers in the Yucatan, it's completly flat. Rain water seeped through the jungle growth, through the limestone and was trapped in the rock underneath. Over the centuries the water eroded the rock and the ground above collapsed. That's geologic history. But centuries ago the natives didn't realize that everything was interconnected. A cenote used in ritual sacrifices was the same water that downstream was used for drinking. The practice of throwing bodies into their water source helped lead to the downfall of the Mayan people.
Today many of the cenotes are still used for water and many more are a spot for recreation. From snorkeling to diving, people enjoy the crystal clear water and the cool temperatures. The Blue cenote has a complex built around it that allows easy access to the sinkhole. Stairs in the rock in an undergound tunnel lead downward. Ladders in the rock help you enter the water if you don't feel like jumping.
There's even a platform high over the water if you are really brave for a dive. A 'lifeguard' watches over it and you can rent life jackets if you're not comfortable with your swimming skills. The water is much cooler than the ocean, taking on the temperature of the surrounding rocks. It can be quite the shock when you first hit it.
Roots from the plants above strain toward the water and there are signs proclaiming 'don't pull on the plants.' A type of catfish live in the water, dark black and totally oblivious to the humans splashing around. Water seeping through the overhanging limestone rains down in small waterfalls.
The facility around the cenote includes showers, lockers, a restaurant and a gift shop. Everything is spotless with attendants cleaning and assistants for lockers and towels. Of course there is a tip jar everywhere, but a few pesos dropped in a bowl is a small price to pay for such wonderful facilities in the middle of nowhere.
If you plan to swim for any length of time I would recommend a shorty wetsuit. The water gets cold fast. I had a 'sharkskin' suit with me and was very happy to have it after a few minutes.