There is a science fiction story by, I believe, Isaac Asimov which has as it's main story point that:

- fusion is developed

- b/c of fusion, the cheapest "rocket fuel" is basically water heated into steam by the fusion reactor

- people on Earth get upset about the "spacers" taking all of the "earthers" water

- the spacers then have to go to other sources of water in the solar system (I think it was either rings of Saturn or the asteroid belt)

The thing that struck me at the time was the "water as propellant" without the extra step of breaking H2O into hydrogen and oxygen.