Cancun city water comes from deep wells. It is filtered and chlorinated at the well and then re-purified by many of the hotels. When it leaves the well it's at least as safe as your home tap water, which is not saying much, given the current state of most community water systems, and all the things that can happen from the well to your faucet. As a visitor, you should stick to bottled water.
You can never be too paranoiac about water in the tropics. Mexicans of all classes agree that tap water is to be viewed with extreme suspicion. Many families, especially with small children, do not trust even bottled water and boil all drinking water for thirty minutes. In the larger hotels, tap water is re-purified and quite safe.
If you are here for any period of time, however, the expense of buying purified water can add up if you buy it by the liter or gallon. It's much less expensive to purchase purified water in 20-liter containers called garrafones, but you will have to pay for the garrafón the first time. After that, you just exchange it when you get water again. The best brand is Cristal. Boiling water is very definitely recommended if bottled water is not available. You can also use Microdyn sterilizing solution, available in all supermarkets and pharmacies.
Colloidal silver water sterilizing solution
When feeling insecure, we also use these drops before filtering. To purify contaminated water, put one drop in a quart of water and let sit for twenty minutes. Good for soaking vegetables and fruits before eating. For very contaminated water, let sit for three hours before using. Don't use metal utensils or containers.
Fruits & Vegetables
Hotels Ice Cream
Zona de Tolerancia
Words to live by.