Two things on water:
1) It is quite stable in the atmosphere. If too much vapor is present, it condenses and falls as precipitation.
2) Water vapor is indeed a powerful greenhouse gas. Since evaporation is directly proportional to temperature, there is a huge positive feedback - anything that raises temperature raises evaporation, which hugely raises temperature. That is one of the reasons the relatively weak greenhouse effect of CO2 is so important. Similar effect for methane - warming thaws the permafrost, which releases methane, which hugely aids warming. The focus is on CO2 because it is a controllable emission with huge climate leverage (due to the feedbacks described above).