Water in Mexico
The reason you can’t drink the water in Mexico has less to do with the way it is treated and more to do with the distribution system. Water only runs for a few hours per day and it goes into a cistern under your house. A pump carries the water up to a container on the roof to give you pressure. So the water has to sit in the cistern and again on the roof. By the time it actually comes out of the tap, it may have been sitting there for quite a while. (At this this is how it is in Durango Mexcio, and I think most other places in Mexico.)
The cisterns and holding tanks don’t seem to be something that gets cleaned very often and you run the risk of a mouse or something crawling into the holding tank and drowning.