You're absolutely right, via natural causes. Now you intend to super saturate it via artificial means - burning hydrogen. Rain you say? Quite likely and a hell of a lot of it...........
Nothing like increased water vapor in the atmosphere to increase the greenhouse effect......
The amount of extra water vapor put in the atmosphere if every vehicle were driven by fuel cells is infintesimally small compared to natural evaporations from the ocean. I wouldn't lose sleep over it.
And we can't "super saturate" the atmosphere.