Many racers use straight water because water conducts heat better without antifreeze, and since a racing car is moving fast enough through the air, the radiator cools the water below its natural boiling point. A lot of people seem to think that water isn't that good a fluid to cool things with. Granted, there are fluids with better thermal properties than water, but water is pretty darn good at transferring heat. After all, we used water in the Navy to cool our nuclear reactors, so it can't be that bad. Oh, and we only used oil to cool bearings and other parts where the oil actually lubricated - oil is NOT a good choice for cooling.
Water is simple, cheap, and natural, and pretty easy to clean when it spills.
Seriously, though, some have suggested alcohol, which might actually be better. Alcohol cools the skin quicker than water, I think due to its low evaporation point. As for actually transferring heat, I do not know for sure. Someday I may try it. For now, water is it.