In order for H2 cars to be truly green, you have to take water and split it and vent the oxygen. So you get the water back when you run your fuel cell car, and all is right again with the world.
Now all we need is a way of turning sunbeams into electricity, and we're all set. Of course, you could just charge up batteries instead, much more efficiently.
You can't burn coal to make the electricity without blowing your green cred. Using oil, natural gas, etc. to make hydrogen, is even worse because they're fossil fuels and also increase CO2. .