A number of websites and mail-order organizations promote ways of using water as fuel or to assist in fueling a vehicle. This is an appealing idea, but also a very old and misleading idea. To use water as a fuel is not only unproven, but actually impossible according to the laws of physics - in particular, the first law of thermodynamics.
For more information, see Zero-point energy and Wikipedia:Water-fuelled car.
Water injection: established technology
Note however that water injectionW for cars is an established technique that does not use water as fuel, but rather as a way of cooling the intake, increasing the effective octane level of the fuel, and thus allowing greater compression.
