One of the most strikingly beautiful swimming spots of the Yucatan is Ik Kil Cenote- a natural sinkhole near the Mayan ruins of Chichen Itza. The cenote (sometimes called Ikkil Cenote, Cenote Ik Kil, or just plain Ik Kil), is part of the Ik Kil Archaeological Park, and is a popular tourist destination.

The pool of water in the cenote is located 85 feet (26 meters) below ground (the pool itself is 40 meters or 130 feet deep), and is framed by rays of light, vines, vegetation, and small waterfalls. Visitors can get to the pool by walking down a carved staircase, but some folks choose to jump or dive in- at lest on special occasions. Ik Kil was part of the Red Bull Cliff Diving World Series in 2011 and 2010!
Though normal visitors don't have the opportunity to jump from the mouth of the cenote into the center like professional divers, there is a platform inside the cenote (maybe 15-20 feet up) from which people can jump into the water (I highly recommend it).

You can only use biodegradable products on the reef and in cenotes. Biodegradable AfterSun, Sun Block and Insect Repellent are available when you arrival.
Option: If you want to see the Light & Sound show, your pickup time will be 11AM in the winter or 12PM in the summer. This gives you almost 3 hrs in the ruins before the show. Light Show Hours: 6PM - 7PM Winter; 8PM - 9PM Summer.
All sites of ruins close at 5PM. Plan your time accordingly.