The short answer to this question is no. It is not possible to run a car on water, and it never will be, unless fusion technology becomes commercially viable, and even then, an external energy source would still be required. Numerous scam artists have attempted to convince people that it is possible for a car to run on water, and they are often happy to provide people with expensive guides to show the steps along the way, but what they are promoting is a perpetual motion machine. Were it indeed possible to create such a machine, the world's energy problems would be solved, but don't hold your breath waiting for such an invention.
Typically, the claim that a car can run on water goes something like this: water is poured into the fuel tank of the car, and some magical unknown energy source converts the water into hydrogen and oxygen, which are burned to generate energy for the car. The waste product of the burning, miraculously, is water, which can be cycled through the system all over again. Anyone with a rudimentary understanding of the laws of physics should be able to discern that it is not possible for a car to run on water using this system.
There are a number of problems with the claim that it is possible for a car to run on water. For one thing, such systems assume 100% efficiency, which is essentially impossible, as it would imply that a car was capable of regenerating energy endlessly. Fuels like gasoline generally run at around 14% efficiency, with hybrid technology being slightly more efficient, thanks to the use of recaptured energy from braking. Were someone to develop technology which ran at such a high rate of efficiency, it would be cause for global celebration, and widespread scientific skepticism, because it is generally believed to be impossible.
For a car to run on water, it needs some sort of energy source to convert the water into a usable form of fuel. This means that it is physically not possible for a car to run on water alone; the oft-touted “hydrogen conversion kits” sold for cars in fact run on fuel, which is burned to generate the reaction which converts the water into usable fuel. Which such systems may be more fuel-efficient than conventional gas engines, it would be a bit of a stretch to claim that a vehicle outfitted with a conversion kit ran on water.
Some people are optimistic about using fusion, which could be harnessed for a car to run on water. Fusion requires the generation of a reaction which causes the nuclei of atoms to fuse together, becoming extremely heavy and generating energy in the process. While an external energy source is needed to trigger the fusion reaction, a relatively small amount of energy could generate a large return, making fusion a potentially very efficient and attractive source of energy.