You MUST put energy into water to split it into hydrogen and oxygen (e.g. electricity from a battery: the well-known process of electrolysis) which could then be run through the fuel cell to produce...electricity. The first law of thermodynamics allows no system whereby you can get more energy out than you put in, in fact you'd inevitably waste some as heat, so why would you put a fuel cell in between your power source (e.g. battery) and the motor? It'd be a lot more efficient to just run the car off of the battery, not to mention saving the expense of a fuel cell, and the weight penalty of lugging it and the water around.

If water is the only source of power, you could run the exhaust pipe into the fuel tank and run forever without refueling - another perpetual motion machine in hi-tech guise; undoubtedly a scam.