[KTM rider]

Oh boy here we go !, some returns into water by the combustion in the motor but in the process most water will not be replaced, EVER, most of the water coming out of the tailpipe will be atmosheric condensation, (oh look ma see no problem) , it takes a lot of energy to turn the hydrogen back into water, it is a losing game, a big big loss.

It actually takes more energy to produce the hydrogen than the hydrogen produced can provide and most of the water destroyed will never be replaced, ever

maybe Elon Musk can go find a chunk of ice on mars and tow it home

[NorthMountain]

it takes a lot of energy to turn the hydrogen back into water,

Hydrogen just loves to give up energy by oxidation. You have the problem backwards: It takes a lot of energy to crack water into oxygen and hydrogen.

And yes, it's a stupid idea for several reasons. "Losing water" isn't one of them.