The far and extreme ultraviolet (FUV/EUV) solar flux determines the photodestruction rates of H2O and OH in cometary comae and affects their spatial density distributions. Between the minimum and maximum of the solar activity cycle, the solar flux varies by as much as a factor of 2-3 at some FUV and EUV wavelengths. From recent cross section measurements, a solar spectrum model using the He 1 λ10830 equivalent width and 10.7 cm flux as solar activity indices, and estimates of destruction rates due to the solar wind, we can estimate the H2O and OH lifetimes appropriate to any observation since 1975. The calculated H2O and OH lifetimes against direct photodissociation both vary by 30%, though the additional destruction of OH through predissociation accounts for most of the 70% variability in its total lifetime. Lifetimes for H2O and OH corresponding to the solar activity minimum and maximum are presented along with parametrizations to estimate lifetimes appropriate for any observation date. As a by-product, we calculate the time-variable yields for producing OH(A2Σ+), O(1D), and O(3P) from H2O and OH. The H2O and OH lifetimes were included in a vectorial density distribution model and tested against International Ultraviolet Explorer (IUE) spacecraft observations in which the OH coma was spatially mapped, and the agreement is quite good. The implications of this new OH modeling on the interpretation of observations are exemplified in a brief analysis of QH2O for comets Bradfield (1979 X) and Austin (1990 V).