the whole idea behind the recommendation that you should only use distilled water in your batteries is that there is no other common standard that can insure that the water is "reasonably" clean.... and reasonably clean water is all that is needed.
reasonably clean water helps insure the maximum longevity of the battery by limiting the degradation from minerals that are detrimental to the workings of the battery.
all across the country, and even different locations in the same vicinity, the water quality can vary so much that it wouldnt be prudent in recommending that you just add water from your local source, whatever or wherever that may be.
if you have a choice between running your batteries with the plates showing and bare, or adding river water (NOT salt or brackish), i would highly recommend covering the plates with river water.... bare plates are more dangerous to your health and the healt of the battery than is the river water.
(disclaimer; I am assuming the "river water" is clean enough to routinely swim or bath in)
but that scenario would mean you have neglected the service intervals on the batteries... if you maintain regular service intervals, you probably have acceptable water available.
for the first 20 years of my life, all i knew was to add water to the equipment and vehicle batteries when it got down near the top of the plates.... we had natural spring water.... we never had any unreasonable battery problems.
but our neighbors up the road a few miles had a well.... it had either sulfur or iron in it and it smelled up their house when the wife did dishes or anyone took a shower.... this type of water would never do in a battery.
some municipalities (towns,cities) have different minerals in their water... its regulated (questionable) for human consumption rather than for battery use.
ALSO, a big thing here.... how is the charging system? how often do you have to add water?... some batteries may need water once a month, and some batteries may need water once in their long extended life..... how many times and how much water you add each time will determine the percentage of harmful minerals are accumulating in any given battery....
all these variables cannot be accounted for when a battery company or battery "scientist" recommends that you add water, but they do know that reasonably clean water will be better than some of the water that we currently consume in our own bodies.
by recommending distilled water to maximize the life of the battery is a good "blanket coverage" statement for anyone who doesnt know the quality of the water they have available.
there is no water that we would want to drink that will "kill" a battery immediately by adding some when needed.... now it may take time from the full life of the battery, but so will over charging, discharging too deeply, letting the water get so low the plates get bare, and inactivity of the battery over a long period....
with all that said, those of us who are in the business of taking care of other peoples batteries for them, we must use distilled water for liability reasons in the event of a premature battery failure (and they commonly do).... but in my own batteries i dont discriminate.... regular bottled water has always served me and my batteries well.
