Because of the limestone geology of the Yucatan Peninsula, there are hardly any rivers in the area. Instead, rainwater percolates quickly into the limestone aquifer beneath. Ancient humans who settled in the area, would have relied on access to fresh water, which made cenotes super important.
Think about other areas of the world (South Florida, for example) and discuss the availability of fresh water in that area. Is it plentiful, scarce, pure, polluted?