theres two different mix types you can run
75/25 (75 water 25 coolant)
or 50/50
either way you have to run coolant because water will just boil off like mirrorimg said, water wetter, and other cooling additives are merely an oil based additive designed to prevent air bubble formation against specific areas inside the engine, air bubble formation against the coolant passages, regardless of how well bled your system is, causes hot spots, which causes failures. I use all premixed 50/50 peak coolant, drain the radiator once a year and refill, keeps a constant flow of good coolant in there on a yearly basis. The only time i would worry about using water wetter would be if you were doing hardcore track racing where a consistently high coolant temp is normal, and you cannot afford to have a hot spot.