I'd hope that hydrogen from natural gas would be a temporary phenomenon - my impression is that the gas is quite likely to run out sooner than the oil anyway.

No doubt someone has done the sums on the water vapour issue, and clearly they should. My guess is that the figures would be fairly reassuring about the possible impact, but guesses like that aren't worth much.

Agreed, it's not a good idea to assume that any technical fix will make it possible to keep on racing ahead and using more and more energy. There are definitely limiting factors. And there are potential drawbacks to pretty well all alternative energy sources that need to be taken heed of.