the locals' digestive tracts are accustomed to the bacteria that makes you sick. its not necessarily that the water is bad, its just ur body wont be used to it. but like i said, when ur down there ull see a lot of places physically have their own water tower. ive never heard of anyone that got sick from a hotel when they had their own water supply like that. however, another thing u should make sure to keep in mind is ice! if its made from mexican water, youll get sick. other than that, dont worry and have a great time!