ddogg777 said:If his system is pressurized he won't need to worry about that. What he needs to worry about is corrosion.
We all know what happens with rust when you just run water.
My thinking is more about higher boost levels and very high temp around the chambers of the cylinder head. That is where you can get boiling water and air pockets. Just plain water will not remove the heat from around the chambers as well as coolant can. This can cause detonation and lifting of the head. He is running hotter cylinder head temps with water then with running water/coolant mix. Not what you see on the temp gauge but around the cylinder head. I would use something like Evans Cool in a high boost engine. We have been able to run high boost with less timing retard with this stuff then with just water or coolant.
http://www.evanscooling.com/main11.htm