A car can run on water but not directly.
A car cannot run on the water alone although the water can be used to make and produce hydrogen gas through a process called electrolysis.
The hydrogen gas that is extracted from the water through electrolysis is what is then burned to power the car.
Water itself is not a fuel as water molecules also known as H2O are chemically stable and don't burn readily like gasoline or other fuels do.
In order to use the water as a fuel source for the cars engine you first have to extract the hydrogen gas from the water through electrolysis.
The electrolysis process to extract the hydrogen gas from the water requires a significant amount of electrical energy so it's not feasible or practical to use water to run a car.
The amount of energy that is required to extract the hydrogen gas from the water to power the car is more than the energy that you would get back from burning it so it would be very inefficient to run a car on water.
The way a water powered car in concept would work is that the water would be split into hydrogen and oxygen gases through the process of electrolysis which uses electricity to break the molecules in the water down.
The hydrogen gas once extracted from the water could then be burned in a combustion engine or used in fuel cells to produce electricity that can power the car.
Some concepts also involve the use of a hybrid system in which the car runs primarily on gasoline but also has a small electrolysis system to supplement the fuel with the hydrogen.