As Loosehead says water vapour is a greenhouse gas. It's complicated but if the atmosphere becomes too saturated with water vapour you get rain and there's less water vapour in the atmosphere.
Unfortunately as the temperature rises the atmosphere's capacity to store water vapour rises. Some believe Venus' water contributed to it's current state:
http://www.astro.washington.edu/larson/Astro15 0b/Lectures/Venus/venus.html#atmosphere
Remember that as well as CO2 fossil fuels also produce water vapour now. and although I don't have the figures to hand I'd imagine that human production of water vapour is likely to be nothing compare to a warm day over the Pacific.
Even so hydrogen is by no means a shoe-in for the fuel of the future role. There are a lot of storage and safety questions that need good answers and it faces strong competion from battery technology at the low end.
In other words I could imagine a world with cars running on battery power by not container ships doing the same.