It is thermodynamically impossible to run a car “on water”. Water is H20. What chemical reaction could water undergo with the air that would liberate energy? The air is mostly N2, 02, CO2, H20 vapor, and Ar.
While water can react with the above chemicals, none of those reactions is exothermic. Meaning, it takes energy to make water react with nitrogen.
Contrariwise, when you react octane, C8H18 with 02, you get CO2 and H20 plus a bunch of energy. This energy is used to drive a piston, which turns a shaft, which turns the wheels of your car, and makes the car go forward.
With me so far?
So there is no known chemical process whereby water can react with elements in the atmosphere to release energy. If you want to convince me that such a process exists, you’ll have to demonstrate it. Since I have no good reason to suspect that such a process exists, in fact, I have good reason to suspect that such a process does not exist, I am justified in claiming that your process does not actually exist unless I can observe that demonstration.
Now, there are plenty of engines that USE water. Steam engines, for instance. Or we could imagine an engine that reacted metallic sodium with water to produce energy. But in those cases the source of energy is not the water. In the case of the steam engine, the energy comes from coal. In the case of a hypothetical sodium engine the energy comes from the sodium, and it would take much more energy to purify the metallic sodium than you could possibly get back by reacting the sodium with water.
Or you could claim that the engine is a fusion engine. Except then it wouldn’t be the water that’s the fuel, but hydrogen. Except we have good reason to suspect that low temperature fusion is impossible, and anyone who claims it IS possible will have to demonstrate that it is possible. Just asking people to prove it doesn’t work is impossible, because none of us knows how it works…because likely it DOESN’T work. I can’t prove you don’t have an invisible fire-breathing dragon in your garage, but I suspect that you don’t. But if you won’t let me into your garage, and won’t let me test for invisible dragons, I have no reason to listen to your claims.
See how that works?