Cozumel has over 220 freshwater wells in the center of the island operated by CAPA that provide water for the municipal water system. This water is chlorinated at a plant near the wells, on the Transversal. From there, the water is fed to the city in buried asbestos/cement pipes; fragile pipes that are full of cracks and leaks. Unfortunately, these cracked pipes were buried in trenches with the sewer pipes, so since the fresh water pipes are low pressure (and sometimes no pressure) the sewer water can infiltrate the freshwater pipes. So, nobody drinks the water from the city water system, but use bottled water instead. No water comes to Cozumel from the mainland, except retail bottles of national and international brands in small bottles.
There are several companies on Cozumel that produce drinking water for resale using reverse osmosis and other methods. Most large hotels have their own reverse osmosis systems.
Read more about the water system, the island's hydrology, and CAPA's wells in my new book, NATURAL HISTORY OF COZUMEL, available on Amazon.com