In this type of water cooled engine application, pure water is the most efficient coolant. In contrast to a 50/50 mixture of water and antifreeze, pure water has over twice the thermal conductivity and nearly twice the heat capacity. Said another way, water in the cooling system is capable of transferring twice as much heat out of the same system as compared to a 50/50 glycol coolant and water solution.
In summary, engine operating temperature will be reduced when using pure water as coolant. Temperatures can be reduced even further using a small amount of surfactant additive. Race cars, for example, use pure water plus surfactant if the series rules allow. Otherwise, pure water.
The primary downsides of using pure water as coolant include increased galvanic activity (read: corrosion) and freezing below 0'C.
More technical data is here:
http://www.redlineoil.com/content/files/tech/WaterWetter%20Tech%20Info.pdf
Wrooster