Pure water is essential to good beer – and brewers pay scrupulous attention to the source and purification of their brewing water. The water used in brewing is purified to rigidly set standards. If it doesn’t have the proper calcium or acidic content for maximum activity of the enzymes in the mash, it must be brought up to that standard.
By far the largest component of any beer, the water you use affects flavour, but for most purposes tap water is fine.