UFO's

[Succubus]

If ET is using water as a reaction mass then they will never leave their home system.

The Tyranny of the Rocket Equation.

There is no real difference between using water as reaction mass or using some gas like helium as reaction mass.   You can build very efficient ion propulsion by vaporizing, dissociating water, and ionizing the oxygen and hydrogen and use them as reaction mass using electric propulsion...

I see no net gain in chucking ionised water (for want of a better term) out the engine nozzle as opposed to chucking out superheated steam.