It is actually possible to make a car look like it runs on water without breaking the first law of thermodynamics. The way it's usually done is with metal hydrides. These react with water to produce hydrogen, which is then used to power the car. But since these hydrides will deplete with time, they need to be replaced and so they are actually the fuel, not the water. And you can be sure that more energy will go into producing them than will be taken out, making them an energy carrier, just like a battery.