I guess the question is where you source your hydrogen from, if you are splitting water to get your hydrogen then the release of water back into the water cycle shouldn't make all that much difference in the grand scale of things. But the vested interest might be to source your hydrogen elsewhere.
With carbon being the demon du jour theres a lot of talk of extract hydrogen from fossil fuels (which are hydrocarbons), capturing the carbon and sticking it back underground. But just as the issue of emission of CO2 gets confused because people confuse the natural cycling of carbon with the introduction new carbon into the atmosphere, introducing 'new' water into the atmosphere potentially raises the same problems, its just that at present its an easy sell to suggest that an emission is 'only' water.
CO2 used to only be CO2.
I think its a non-advance. We're too eager to be sold something that saves us having to make any real changes, we're all ears for anything that allows us to just get on doing the same old thing without the guilt. Thats why Top Gear are so unquestioningly sold on it, because they are so deeply sold on all the fantasies of motoring. Its us, not fuel that needs to change.
Edit - I typed that too slowly. nmmmmm boosze