Even if one does use advanced coolant cycles of some sort, water is extremely effective as a coolant. It has one of the highest volumetric heat capacities of a

Even if one does use advanced coolant cycles of some sort, water is extremely effective as a coolant. It has one of the highest volumetric heat capacities of any pumpable liquid at low temperatures, a very low viscosity and manageable reactivity, and a very low-temperature, high-energy phase transition that we understand well for higher temperature applications, which does not even require exotic pressure conditions to achieve. Water is likely to show up at some stage of the cycle in any complex thermodynamic infrastructural engineering.