but wont this just create another problem, the mass for the 'spent' hydrogen would be 9 times more than original because of Oxygen being much weightier (atomic mass: 16). so your fuel tank would basically get 9 times heavier. that will negate any benefit you get from higher energy density. also it may create storage problems because now you have to provision another tank for water storage.

I think they'll just handwave it and release it periodically.