Additives don't make water "cooler". People add things to water to make it less corrosive, decrease surface tension, prevent microbial growth, and lower the freezing point/raise the boiling point. Of these, decreasing surface tension can increase heat transfer and more efficiently cool the cpu.
Lowering the freezing point is only needed when active cooling (peltier, refridgeration) is used. Most water cooling kits are passive, so there isn't an issue of being "too cold".
If you are operating in a very high temperature environment, these passive coolers won't cut it, you'll have to look at peltier cooling, or a vapochill (big $$$).