You can purchase bottled water everywhere you go in the Yucatan except some of the long barren highways. Any road to any attraction, or ruin site will have stores along the way.

All the resorts have their own water purifiying systems. Some say that this water is okay to drink, some say it is not. If you're only staying for a short period you can find bottled water anywhere, so drink that. Use your hotel water for brushing your teeth if you wish and taking showers but DRINK only bottled water. Almost every resort and hotel offer 1 or 2 bottles of water per room per day. If not you can purchase it in the resort/hotel store or in a nearby store.

Ice is safe. The ice that is provided to most restaurants and bars is produced in ice factories using purified water. Hotels and restaurants that use ice machines also use purified water. Even the guy's downtown selling home-made popsicles use purified water.

Drink bottled water, brush your teeth with sink water :) Safety first.