yea salt water that would corrode the hell out of an engine and all parts in between. desalinization isn't cheap enough yet, at least not for fresh human drinking water, but if they'd truck in desalinized ocean water for cars im all for that. Just stay out of my drinking supply
2. The byproduct of using hydrogen in an internal combustion engine is H2O.
which doesn't directly fall back to the ground and soak back into the aquifer, even if such a miracle did happen it would take a lonnnnng time to make its way back, it would also have to rain directly on a recharge zone.
im in FL... people keep moving in, people keep building houses, and they keep sucking the ground dry. Sink holes don't happen for no reason, and this is in a state the gets more then its fair share of rainfall.