Depending on the tap water you may be introducing potentially harmful minerals to the mix. Distilled water has essentially no minerals, so the only chemicals other than water in the system are the inhibitors in the antifreeze.

Most manufacturers recommend mixing antifreeze with "potable water", but that covers a wide range. A few say "distilled water", and you can't go wrong with it.

Bear in mind that the recommendation for distilled water is mixed in proper proportion with a proper antifreeze (MB brand or Zerex G-05 for Mercs.). Straight, 100 percent tap water or distilled water, without proper antifreeze in the proper proportion will cause rapid corrosion in the cooling system.

Duke