“The ecosphere is already saturated with water”
So, you are walking around in a dense fog right now? As is everyone else on earth?
Average Global Relative humidity at the surface is 78%. At higher altitudes it drops, being 37% at 30,000’.
Correct.
However, since liquid water is widely exposed to the atmosphere, the system is at equilibrium. Liquid water evaporates and water vapor condenses. The addition of a miniscule amount of water vapor will not change these conditions.