Here is what Gooch has to say about using tap water, versus distilled water, in our engines.

The following information was compiled from several posts in a single thread, by Gooch.
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Even some of the best tap water will contain undesirable minerals, dissolved salts, and acids. Those components, like calcium, iron, magnesium, carbonates, hydroxides, nitrates, and so on, not only reduce the life of the coolant, but they reduce its ability to protect the cooling system. Those of us experienced with cooling systems know that tap water undoubtedly leads to exacerbated cooling system failures.

Since tap water is not pH neutral, it changes the desired alkalinity of the coolant formulation when mixed with the coolant. Distilled water, although not always completely mineral-free, is generally more neutral than reverse osmosis and purified water.

By using proper cooling system maintenance, you won't need to worry about having your radiator rodded or replaced.

You could flush with tap water, as long as your tap water was exceptional. Otherwise it is best to make your final flush(es) with distilled water. Always fill with distilled water.

It doesn't matter what conditions you operate your vehicle under. Coolant with distilled water will always protect your cooling system better and longer than tap water.

Using tap water for coolant maintenance...is second-rate maintenance. That is not opinion, but a well-known fact.

Where coolants are concerned, the facts are the facts. Using tap water will have consequences down the road...reduced protection, reduced coolant life, increased maintenance, and premature repair. Using tap water is sloppy, second-rate work.
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Stewart