[trhway]

Due to incompressibility of liquids, pressurizing a liquid is very cheap energy-wise - orders of magnitude cheaper than pressurizing a gas. The issue is that pressurized liquid also requires correspondingly strong and expensive vessels and pipes.

[sfn42]

You don't need to pressurize it. You just put it in a tank and apply heat, the water evaporates and creates pressure. Like a pressure cooker.