EcoGeek has a nice article that dispels the widespread myth that cars that can run on nothing but water are on the horizon. The basic laws of chemistry tell us that to get water powered cars we’d have to build the world’s first perpetual motion machine.

As they put it:

There are a lot of variations on the water powered car, but they’re all bogus…. you can’t get more energy out of the system than you put in.

In other words, powering a car with nothing but water violates the laws of chemistry. Energy is needed not only to break water into its hydrogen and oxygen parts but also to recombine them. The only conditions under which this process can take place are those with highly engineered systems, with the built-in energy to make this process happen.

However, the process cannot take place with the presence of water alone.