The clearest natural fresh water lake in the world is apparently Blue Lake, near Nelson in New Zealand.
I'm wondering how one would reach it - the tracks to take, walking times etc - ideally a map and directions would be a valid answer.
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Wikipedia's entry on this lake has detailed information on tracks and lodges. It also provides 6 references with details. Apparently about 700 people visit annually. It is reached as a side trip of the Travers-Sabine circuit.
DISCLAIMER: I haven't been there, all information stated here is fully based on the mentioned Wikipedia entry.