looks like the standard forcing/feedback mistake in your post
If I understand where you're heading, it's a variation of "water vapor is a much more important greenhouse gas, so doubling CO2 doesn't matter."
Water vapor is a feedback that changes as a result of CO2 forcing due to human activities. Read this:
http://www.realclimate.org/index.php?p=142#more-142
You also said:
"CO2 pound for pound [or in terms of liquid or gas volume] is not a substance which is as effective greenhouse agent as H2O"
You didn't want to bet me on global warming - how about a bet as to the accuracy of that statement?