Only bi-product on the buses is 100% pure water.
I'm curious, does it come out of the engine as water or steam? Where does it go? Water vapor is a more significant green house gas than CO2.
A better idea may be to place induction coils in the road beds of highly traveled routes. Cars could use this power on the freeway then use battery power during the last mile to the destination. There are no vehicle emissions in that, not even water vapor.
The hydrogen powered bus produces only water vapor. But what about the process that seperates the hydrogen from whatever it's bound to?