I think Marshall Will's right about the dowsers, I don't know if you've ever seen one in action, but it'll make you scratch your head the first time or two. I know several people who witch wells and have for years and people seek these people out before drilling a well, when they say it's right here, that means right here, not in this general area. a few feet can mean the difference between wet or dry around here.
as for the water used on the trail, true it was pure conpared to the toxins we as people have brought to it over the years, but there were millions of Buffalo to pollute the streams and rivers. I think as the pioneers and Indians moved across the country, their immune systems adjusted to the type of water, they were drinking and built up a natural immunity. As a kid in Oklahoma, we drank out of every little spring we came too. nobody got sick but we had done it since we were small.
I really doubt that when the cosie went out and got water he brought it back and and pre-boiled it. he had other things to worry about. like getting supper ready.