I think there's like 3 orders of magnitude more water in our atmosphere by weight than CO2, so a swap wouldn't drown us. But the point you were trying to make is clear.
The problem with H2O wouldn't be the absolute amount, but the changes brought on by the added H2O. For one thing, it would rain pretty much 24/7 in Florida. :) (Yes, I know it's not at all that predictable.)