Don't drink the water... part 4,687,312

Amazingly, this real post from a tourist message board elicited mostly serious, thoughtful replies -- 22 of them!

My sister is going to Mexico to stay at a time share type place with friends. She is limited as to what she is able to drink (non alcoholic). No soft drinks, no carbonation, no caffiene and no alcohol. Is plain non mineral bottled water available most places? Any US brands available - not US snob, needs to be able to read that it is plain in english.