The specific impulse does not take the engine (or reactor) mass into account, but it's (approximately, if you neglect pressure effects) proportional to the exit

The specific impulse does not take the engine (or reactor) mass into account, but it's (approximately, if you neglect pressure effects) proportional to the exit velocity of the gas flying out of the nozzle. The exit velocity in turn is proportional to the square root of (temperature divided by molar mass of the exhaust gas). Thus, if you use cooler hydrogen gas rather than hotter water+hydrogen, you can still make it a lot more efficient given that water has 9 times the mass of hydrogen per molecule.