It's official the clearest, cleanest, freshest water is in New Zealand at Blue Lake. This according to the National Institute of Water and Atmospheric Research. Wow, take a look.
It's even compared to distilled water, “The theoretical visibility in distilled water is about 80 meters, as estimated from the best available instrumental measurements in the laboratory. So Blue Lake is a close approach to optically pure water”. I want to take a dip. More here.