"Water Wetter" is designed to reduce hot spots in your cylinder head. It does this by reducing the build-up of water vapor in any superheated areas. The bad thing about having hot spots in your cylinder head (read combustion chamber) is that they can promote pre-ignition - definitely a bad thing. This harks back to Smokey Yunick's theory of "soft combustion chambers". Any sharp edges in your combustion chamber (around valve seats for example) may tend to get very hot (even red hot) during operation. These areas of the combustion chamber can then form local hot spots in the cooling passages. Thus, even though your bulk coolant temperature is well below its boiling point (i.e. your gauge reads just fine), there may be localized boiling in some regions of the coolant tract.