We want to make sure several million cars spewing out pure water vapor in an urban area doesn’t create unintended local climate effects.
OK, I thought you were joking. Water vapor is the “greenhouse” gas that accounts for literally 98% of the “greenhouse” affect on earth. But even many millions of cars wouldn’t emit literally even a drop in the bucket as compared to evaporation from the oceans and bodies of water which make up over 71 percent of the earth’s surface, not to mention that a great deal of water evaporates from land areas also. It is unlikely that the amount of water vapor from fuel cells even in highly populated areas would ever be able to increase humidity enough even in highly populated areas to have a measurable effect.
You have however touched on something that is a tenant of the global warming nonsense however. The theory depends on CO2 increasing the humidity of the planet... there is no conclusive evidence that the humidity of the planet has been increasing in the last several decades however.