Some year ago I did archaeology at Canyon de Chelly and we regularly visited an extremely isolated site (Navajo Fortress) that required some fairly serious climbing to access. We needed to carry all our water which typically involved an overnight stay.

On one occasion we stashed an extra gallon of water, returning a year later. Naturally we brought a full ration of water on our return, not knowing the condition of the stash we had left behind. It was ordinary tap water, not treated in any way. As it turned out, it was perfectly fine. The only precaution we took was to store it away from direct sunlight, but it was exposed to both high (90-100F) and subfreezing (circa 0F) temps during the intervening year.

I don't bother with commercially bottled water, except in a few rare situations. There are a fair number of studies that show tap water to be just as good, or even better.