They were used by the Mayas as reservoirs and as places of cult.
The bottom of Ikkil cenote can be reached by normal stairs (very accessible). At the bottom there is a platform where you can practice the swimming and diving, just like the Mayans did in ancient times.
Cancun is on the North-East of the Yucatan peninsula. Chichen Itza is located some 200 km South-West from Cancun. Other interesting Maya sites are found further West, South of the colonial city of Merida. Tulum is the only site on the beach (Eastern coast), which confers it a special beauty.