What water wetter does is to make the water more "wet" so the water covers more surface. This will transfer more heat from the metal and to the water and from the water through the radiator and to the air. Can be used with only water since it inhibbits corrosion and with water and anti freeze. Water has better cooling properties than anti freeze, so pure water and water wetter will be the best option when there is no chance of the water freezing. This is what I use in the summer.