The coolant fluid used in most internal combustion engines is typically a mixture of glycol (for freeze protection), a pH buffer, corrosion inhibitors and water.
Using distilled or deionized water in the solution is preferred because deposit forming minerals (usually calcium and magnesium compounds, a/k/a “hardness”) and corrosive ions (such as chloride and sulfate) are eliminated. However, distilled or deionized water should not be used as the sole ingredient of the cooling fluid as these are corrosive to many metals commonly used in engines including ferrous metals (iron and steel) and cuprous metals (copper, brass, and bronze).
Depending on the source, “tap” water might be usable. In several areas, municipal source water from surface reservoirs (such as Lake Michigan) has relatively low ion concentration. Although not preferable to using distilled or deionized water, using tap water with total hardness less than 50 mg/l and chloride and sulfate concentrations less than 25 mg/l can usually be done without forming mineral deposits or significantly contributing to corrosion. (Most municipalities now post water quality analyses on the internet.)
Regardless of what water is used, it is important to use a glycol-based product made for the application that includes corrosion inhibitors and a pH buffer. A key point is to get a product that is “made for the application”. For example, using a product that is made for use in a building heating or cooling system (which typically contains an alkaline buffer to maintain the pH in the 9.0 - 10.0 range) in an engine with aluminum components will corrode the aluminum.