Pure water is begging for trouble.
The other end of the spectrum there's Evan's waterless Coolant www.evanscooling.com I run the stuff in everything I own. It's already saved two of my engines and two family vacations while towing some pretty heavy loads in high temps.
The stuff has a roughly 375 deg non pressurized boiling point , so you can run your system without having it under pressure, and in the event of a stuck thermostat, etc, your gauge can be pegged for quite some time before any kind of damage could occour to the engine itself ( although I had underhood temps get hot enough to melt clips off the air filter box and fry a few sensors, but nothing that couldn't be expressed in and replaced in 15 minutes once they arrived , no cracked or warped heads or blown head gaskets)

If you are serious about something for hard long distance summer driving, this is the ticket. The price is painful, but I look at it as cheap insurance.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually