Although not technically considered a "fuel" water does contain vast amounts of energy, more so than gasoline.
Proof: E=MC2
The inherent energy in any substance is directly related to its mass. Water, being heavier than gasoline, contains more energy (proportionate to its additional mass for the same volume).
As others have stated, the key is the conversion from mass to energy which right now is done by electrolosis and is quite ineffiecient and then the hydrogen is burned, also inefficient.
Now some fun facts:
one pound of water contains the same amount of energy (if converted 100% effiecently) as 9 billion kilowatt hours or burning 5 1/2 million barrels of oil.
Now if I could just figure out how to....
Seriously, I know some of you will doubt, but the numbers don't lie, and if prompted, I'll bore you with them.