The best thing you could use is DI Water (stripped condensate by way of Deionization). I use it in all my vehichles radiators, and run pure DI water in my Race trucks air/water intercooler. I work at a power plant and this is what we use in making steam. The water's silica, cation, antion, and hardnesses are so low to prevent and buildup or rusting in the boiler, steam drum, or turbine. This water is very pure and clean, its even potable, but i wouldnt drink it, some guys have and it acts like a laxative because it contains no minerals your body wont process/absorb it.

Like you mentioned water has a greater heat exchange compaired to antifreeze, but its mildly currosive. Ive been running DI water in everything i own for several years will no buildup or pitting on aluminum parts.

Now as how one would go about aquiring DI water without working at a power plant?

That i dont know, but i figured i would toss that out there for you to ponder.