Presumably this would be done with a small nuclear reactor powering an electrolysis system, in which case it’s feasible. Your savings are in complexity and weight because containing a mass of water is trivial, and naturally dense compared to H or O. As an extra feature you need water anyway, it can be used as a coolant, and you need oxygen which you’re now generating as a byproduct. Presumably for cruising you’d use the nuclear reactor to power an ion drive and only use the Water splitter as needed for maneuvering, launches, and landing.