Water is the principal combustion product. The problem with hydrogen storage, however, is the fuel tank: when compressed into and placed in steel bottles, like oxygen bottles, it all too readily combines with the iron to form iron hydride, which is very brittle. The only reason the Haber-Bosch process is able to hydrogen bottles for catylizing nitrogen into making ammonia is by enclosing the bottle within another bottle, so that when hydrogen bottle fails, the explosion is contained. Solve this problem, and hydrogen powered vehicles will become practical. For the hydrogen, all that's necessary to to electrolyze water into its constituents.