Nuances of Alum. heads and motor parts.....

[Wild Mango]

On the newer cars they have a motor gounded in 3 or 4 places, one a neg battery ground, then a couple of body to motor grounds, usually at the firewall and at the radiator supports, then usually on the lower radiator mounting bolts, they'll also have grounding straps.
Its a lot more critical on these newer cars we drive now since they are about 1/2 plastic or composite materials.

Yes, thats right on, Screamin'. Still, if the radiator water is conductive the least noble metal in the earthed (bonded) system will still erode, becoming in effect the sacrificial anode. Once the process starts it just gets worse as the coolant becomes more and more conductive as it picks up metal salts.
Distilled water is to all intents non conductive, but anything you put in it from there will increase the conductivity. Keeping the system sealed will keep oxygen out, which effectively eliminates oxidisation corrosion, which again, increases the conductivity of the coolant from the metal salts.
Because we don't need to run anti freeze here, I just use distilled water with a drop of wetting agent, like neutral pH liquid soap and change it out when it looks cruddy.