"Car running on tapwater" is just like humans flying without wings. Not gonna happen.
If you want to have good explanation, drink 5-10 litres of water in a day for few weeks and you still won't gain any weight. Simply there is just no real energy in water.
And I keep hearing one gallon of raw oil has enough energy to feed a man for roughly 40-50 days energy-wise, if a man would be able to use oil as a source of energy for its own body. That's quite hell of a lot, atleast 70,000 - 87,500 kcal. And 1,5l bottle of Coca-Cola has *only* about 730kcal of energy.
And with this miracle energy stuff, sounds just rather questionable that "Hey in water there is an energy source that could save the world we just can't use it" - kinda like asking someone to prove that does some object exist in this world without using our senses to "sense" the object.
Not on water, but running them by using deuterium and/or tritium would be possible theoretically