Water is the coolant. The antifreeze is best thought of as just an additive to lower the freezing point plus provide some lubrication for the water pump.
I think today's coolant is more then just an additive to prevent freezing.
Prestone's mixing advice shows:
50/50 coolant / water = 265F boiling point.
70/30 coolant / water = 276F boiling point.
So if water cooled better then antifreeze, wouldn't more coolant concentrate not do anything for the boiling point?
What really confuses me is Red Line Oil's Water Wetter Super Coolant additive, which states:
50/50 coolant / water = 228F
50/50 coolant / water + water wetter = 220F
water only = 220F
water + water wetter = 202F
So according to these guys, water only is better then 50/50 coolant / water mix.
I don't know who to believe so I run 50/50 mix + Water Wetter for the heck of it.