I actually read something on steam powered rocket engines recently. The main issue is how you actually heat the water. I doubt it would be very useful, at least if it's actually on a rocket. If you're just doing it for shits and giggles then why not?
But from what I've heard it takes a lot of power to use them , meaning lots of batteries. So unless you want the ion engine equivalent of a crappy chinese knockoff, I'd stick to a more standard propulsion mechanism.