For high thrust, you have to make a lot of steam in a hurry. Electrical heating is impractical. Chemical heating works: you burn hydrogen to make the steam, as the Space Shuttle main engines did. A really hot, high-powered nuclear reactor might work.
A bunch of people are working on thissteam for small, low thrust systems. For low thrust, electricity is a good energy source. It's even possible to electrically heat the steam to plasma temperatures to get higher specific impulse. Good for adjusting orbits, but not for thrusting against gravity.
https://spacenews.com/water-propulsion-technologies-picking-up-steam/