AFAIK water vapor is in a thermodynamical balance in the atmosphere and there is not a lot that we humans can do to affect it.

Oceans are producing million times as much vapor as all human activities combined. It would be interesting though to see how GW plays on this - we can expect with rising temperatures the air humidity to grow producing a positive feedback. On the other hand the water cycle (evaporation -> precipitation) will intensify, taking away some of the energy absorbed by the oceans thus cooling them off. I have to check but probably the total efect would be neutral because it is absent in the climatographic models I've seen.

Ahh, but a warmer atmosphere cdan hold more H2O vapor.