Coolant temp can reach over 100, thats why its not just water in the system, the coolant actually raises the boiling point.
so does pressure, if you get the pressure high enough then 130c water is possible.
gas is gas due to the intermolecular forces and energy given to the system and energy opposing the system, conversely in a low enough partial vacumn you could reduce the boiling point of water aswell.
when i read this i was thinking of silent running with the sad ending, never seen cool running.