Science

Just leave it as water, then drop small pellets of lithium in as necessary. Sodium works, too, and is more abundant/available than lithium, but maybe tougher to control safely. (The rest of that group is just too reactive, unless you can find a way to use the exothermic reaction for something other than an uncontrolled fire or even explosion.)

Mostly kidding, but only because I can't imagine smarter people than I haven't ruled it out for very good reasons. And while I'm on the topic, running a condenser on the exhaust will capture the water vapour, which is an extremely powerful greenhouse gas.

Hmmm. I've seen a few references to Toyota supposedly having a prototype system for generating hydrogen from water on board cars. I've dismissed that as just the latest water powered flavour of the month. You don't suppose...