If it is not safe to drink the water in Mexico why is it alright to drink the beer?
In the brewing process, the wort (water with grains and hops in it) is boiled (heat pasturized), which kills off the bacteria. Essentially, all the water used in the process is purified. Likewise, you can drink the purified bottled water in Mexico. We just take it for granted that tap water is safe to drink, but in most of the world, that is not the case.