Generally, tap water is not drinkable, but it can be used for bathing without risk. Bottled water is cheap and can be bought everywhere, even at grocery stores and minimarkets (open 24 h). Purified water is used to make ice for beverages. American and European drugs are readily available at all chemists / drug stores, usually open 24 hours a day, avoiding the need to stuff your bags with your favourite brands. If you are on specially prescribed medicine from your home GP we do advise bringing these with you however.
Vaccinations are not required for a visit to Mexico. We strongly advise that you constantly use repellents to keep away mosquitoes and other bothersome insects, especially during the humid season.