EQUULEUS
Technically, the water isn't the fuel, it's the working fluid and propellant. Not all rockets use a fuel but when they do, it's the energy source and its reaction products are the propellant.
The demonstration of water-propelled engines strikes me as one of those "obvious in hindsight" things that could turn out to be very useful for very cheap smallsats, for which even the use of noble gases is (apparently) not viable.