Water is 75% to 90% of the mash.  Pay careful attention to your water quality.

Tap Water

Forget about it !!!!!!!  Your finished product will taste off.  Most all tap water has many contaminants that alter the taste by being acidic.  Many tap waters contain arsenic, chemical remnants from drugs that were flushed down the toilet, fluoride, just to name a few prominent ones.  

Tap water is highly chlorinated.  When the chlorine mixes with the nitrogen from the mash cholorophenols and other byproducts that taint your final alcohol product.

Distilled Water

The best.  It has a slightly acidic quality that will work well with yeast.  There is no taste whatsoever that will affect the final product.  It give a pureness to the final distilled alcohol.

Filtered Water

This is the easiest and quickest way to get great water.

The best is a Berkey water filtering system.  The Berkey black filter gets the most bad elements out.  The second stage Berkey filter takes out fluoride and arsenic amount other contaminants.