Blue Lake in New Zealand is a naturally clear expanse of water and has visibility to 80 meters, which is as clear as distilled water.
One of the glacial lakes in Milford Sound, NZ
I took this photo of one of the small glacial lakes - it was about 1.5m deep at this point and crystal clear - the aquifers purify the water and keep it beautifully clear - but it's not the clearest water in the world...
The lack of canopy keeps the water clear of debris
The aquifers keep the visibility to up to 80 metres - making it optically as clear as distilled water