I am an old desert rat that has been really thirsty a time or two so I really value water. Basically all of my canteens are filled all or most of the time. I can assure you that lousy water is much better than no water at all. it can almost always be treated so that survival will occur. There are some exceptions, of course, but they are rare....

I would be very cautious about drinking from a water source labeled as "Arsenic Spring" on my trusty topo map, for instance.