Bail the humidifier idea.
I drink nothing exept distilled water. It's by far the best water to consume...human, car, or whatever, and isn't any more expensive than "spring water" for drinking. Most of the supermarkets carry it in the drinking water section. I buy 2.5 gal. containers. Definately use nothing except distilled water for the IC sprayer and radiator. Otherwise, the mineral deposits and corrosion will get you in the long run. It's not protecting the IC itself, it's protecting the spray nozzle and other parts that will eventually get clogged otherwise.
Almost a different subject (I'll probably start a thread), but I've been thinking about using alcohol instead of water. Ethanol cools surfaces more than water. I do believe that rubbing alcohol is flammable, so one should dilute it to probably at least a 70% alcohol/distilled water, so it won't ignite on any hot parts.
Now my other thought would be (for racing) to prepare an isopropyl alcohol with dry ice. Put dry ice in a bath of i-alc. and one gets about a -70 degree solution. You can't do it with water, because the dry ice will boil up in water, but with the i-alc mixture, it doesn't so much. Shooting the IC with a shot of -70 degree fluid would be good. The trick actually would be to have a reservoir in the engine bay, closer to the IC, so it wouldn't warm up during the trip from the trunk. I know Jeg's carries some fuel coolers that would work well. Just a thought.