My knowledge is from 30 years ago, so may well be out-of-date.
Fuel cells need pure hydrogen, which needs to be isolated from water via electricity.
the emissions from a fuel cell is solely water. Also a normal petrol engine can be run from hydrogen (with a few tweeks) and the only emissions would be water vapour.

there are a few mechanical equations which seem enshrined in engineering 'law' for cars.

1. You must keep (unsuspended) mass to a minimum
2. Power = energy consumption (although energy merely changes form)

And engineering is fraught with compromises. Outright straight-line speed is compromised by downforce for cornering. Braking ability, etc.

There are hydrogen powered trains running too