It sounds like you've got a solution looking for a problem. Most of the time an overheat- - -boil over situation on an initial startup is the result of trapped air in the engine block resulting from an improper cooling system filling procedure. I always leave the upper thermostat housing off, and fill the radiator until the coolant level comes up to the bottom of the housing. That eliminates air getting trapped in the cylinder head and creating a hot spot. If you're determined to use some sort of exotic water, just go to Walmart or some other big grocery store and fill one of those 5 gallon carboys with reverse osmosis "filtered" water.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!