Pure water is not considered conductive, but it's impossible to find pure water. Distilled water is ~twice less conductive than tap water. The minerals (less present in distilled water) appear in the water in form of ions, which increases the conductivity of water. The battery effect comes from the potential difference between the two metals. Having a potential difference and a resistance (even a very low one) will cause an electrical current to occur.
But the problem isn't so much that the galvanic corrosion eats away at Alu parts, it's that it leaves a residue in the coolant, which can gum up a loop.
Water Wetter helps to prevent this. Starting with distilled water is always preferable (and cheap, at 59 cents/gallon).
As for anti-freeze... you only need to use it if you plan on using a chiller. Otherwise, stick to the Water Wetter. There are other additives...
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