Absolute line emission cross sections are presented for 1 keV amu−1 charge-exchange collisions of multiply charged solar wind ions with H2O. These cross sections can be used to model charge-exchange processes with cometary targets with similar binding energies such as H, O, CO2 and CO. A parameter-free model is used to successfully predict the recently observed x-ray spectra of comet Linear C/1999 S4 and McNaught-Hartley C/1999 T1. We show that the resulting spectrum is extremely sensitive to the time variations of the solar wind composition.