Not necessarily, right now it's more economical to reform it out of natural gas or other fossil fuel so it's not like hydrogen fuel would take water in that's t

Not necessarily, right now it's more economical to reform it out of natural gas or other fossil fuel so it's not like hydrogen fuel would take water in that's turned into hydrogen. But even if efficient catalyst driven electrolysis were used to generate hydrogen for fuel, it's not likely that the water would be extracted from the air, they'll suck up liquid ground water that wouldn't necessarily have ended up evaporating into the atmosphere. So the water generated by hydrogen burning cars will end up as extra water vapor that wouldn't have otherwise been emitted. But, gasoline burning cars already emit almost a gallon of water vapor for each gallon of fuel burnt, so depending on the efficiency of the hydrogen cars, there could be a net decrease in water vapor emitted. (I'm not sure how much hydrogen a car would use to travel an equivalent distance as a gasoline powered car)