"Many people have “heard” distilled water is best to use in a cooling system. This is WRONG. While it certainly is true that distilled water’s purity prevents electrolysis and scale/deposit formation, it unfortunately comes with a potentially very damaging side effect. During the distillation process, water is vaporized into it’s gaseous phase, so all impurities are left behind. These impurities include a number of minerals, including calcium and magnesium – the two components of “hardness.” The water is then condensed back into it’s liquid phase, so the resulting liquid is pure water – in fact, some of the purest water on earth. The problem is that when water is distilled, or “stripped” of impurities, the resulting solution is composed of chemically imbalanced “ions.” This leaves distilled water “electrochemically hungry,” so it will actually strip electrons from the metals in a cooling system as it attempts to chemically re-balance itself. As it chemically removes electrons from the cooling system metals, it does damage that will eventually lead to leaks and system failure. Using distilled water in combination with antifreeze tends to lessen this effect, as the distilled water will seek and find electrochemical balance from the various chemical ingredients in the antifreeze mixture. However, there is still risk involved.

The best type of water to use as coolant is softened water – especially if you run straight water coolant, without antifreeze. During the water softening process, the same impurities and minerals are removed from water as the distillation process – but with one very important distinction. Rather than STRIPPING the impurities from water, softening EXCHANGES the impurities with a sodium ion. The resulting solution is electrochemically stable and ionically balanced, making softened water very stable, pure, and non-threatening to cooling system metals. It should be added, there seems to be a perceptual issue with regard to usage of softened water. Many mistakenly believe that because SALT is added to water softeners, softened water must contain salt, a substance known to be very corrosive. Nothing could be further from the truth. The salt that’s added to a water softener is NaCl, or sodium chloride. During the softening process, only the sodium ion is exchanged into the water, whereas chloride ions are removed when the softener is regenerated. Therefore, softened water does NOT contain corrosive salt."

That was in the FAQ for norosion.com's website. They're
another company making various coolant additives,
including one similar to Water Wetter.

The problem is that when water is distilled, or “stripped” of impurities, the resulting solution is composed of chemically imbalanced “ions.” This leaves distilled water “electrochemically hungry,” so it will actually strip electrons from the metals in a cooling system as it attempts to chemically re-balance itself.