The problem with water is its low boiling point. There are localized hot spots that are well over average coolant temp which is what the gauge reads. There is never a reason to run straight water.
That's mainly what the Water Wetter products are for. Reduces the surface tension (I think?) so there is less localized boiling. Some think it's a replacement for antifreeze, definitely NOT it has no anti-corrosion or freezing protection at all.
