[ Even if the conversion were 100% efficient, there still remains the problems of storage and transportation. ]
Wrong convert it as you need it.. eliminating storage.. until you need it it remains water.. and the product becomes water again... after burn them together..
Storage of water is not an issue.. its the best way to store hydrogen..
So what provides the energy to convert the water? The vehicle would need to carry around an energy supply equal to the amount to be produced by the engine. 100% efficiency means no loss, not no energy.
Reminds me of Gracie Allen. George saw her putting a pot of boiling water into the refrigerator and asked why she was doing that. "This way," she explained, "whenever I need some boiling water all I have to do is take it out of the fridge and heat it up."