A couple of years ago on this forum (2012), I said that if water were so bad in Mexico people would be dying left and right and everyone would be wearing diapers. I still believe that to be true.
That said, it takes time to get used to a water system. When we traveled through the U.S. and Canada we had water from many different water systems and the effects weren't always pleasant.
Recently, 400,000 people were denied water in Toledo, Ohio because of contamination. We looked around for a water filter but it is all too confusing. Bottled water, if unsafe, would be reported just like any other contamination like e-coli in beef, etc. I've never heard anything.
Here at home we drink from the well and we buy bottled water from a filter system like Windmill in the U.S. Our water is very hard and affects our coffee maker. It also makes guests feel more at ease.
Monterrey has won awards for best water in the Americas over the years. Yes, I know. It depends on the reliability of the system that delivers the water. We pay 10 pesos for a filtered 5 gallon container of water.
I guess each person has the option to make a choice of how they obtain water. There is also the least expensive way, put it in plastic soda bottles and let is set in the sun all day. It kills everything.