Not a bad thought. Hydrogen is the most abundant element in the universe and there better be oxygen around or you don’t need water. Just need to burn the hydrogen, combustion products of burning hydrogen are water and energy.

The hard part would be getting the hydrogen. Then compressing it. Out of reach for the people in the Mad Max world but if you’re in space I’m sure you have a compressor laying around. Just need to event Bussard collectors to harvest the hydrogen.