The Yucatán Peninsula is a 300 km wide carbonate platform riddled with mostly water-filled caves, and with more than 7000 cenotes, dolines with sweet or brackish water inside.
We have a detailed explanation on the page
Yucatán.
The Estado Quintana Roo is the eastern part, the east coast from the northern end to the border to Belize in the south.
State highway 307 connects the cities Cancún and Tulum, and almost all tourist sites are actually located along this road.
The reason is simply that here are the beaches, hotels and infrastructure.
It makes no sense to have a cenote for bathing in the middle of nowhere.
While we listed the cenotes as karst feature and geotope, the actual reason why they are open to the public is not sightseeing, its bathing, snorkeling and diving. As the swimmers are actually swimming in the main aquifer, the cenotes have finally issued some protection rules, which are valid at most sites. They are quite simple:
In other words, it is mandatory that you shower before you go swimming. Also, mandatory is a short briefing from the local staff who will give you the information on how to enjoy the cenote safely. Bring bathing suits, towels and dry clothes. And diving is only allowed in diving schools or with a cave diving certificate.