Fun fact! It is thermodynamically impossible to make a vehicle that derives its energy from water. You cannot get more energy out of a water molecule than you put into it.

Edit: forgot about fusion. It is thermodynamically impossible to derive energy from the chemical bonds in water without putting more energy in than you get out. None of these water powered cars are using fusion generators. They're all about splitting the oxygen from the hydrogen and then burning the hydrogen with the oxygen, and there simply isn't a way to do that without wasting a ton of energy