The detectability of water masers in circumstellar shells with known 1612-MHz emission is examined as a function of their IR colors, by searching every object in the Arecibo sample of OH/IR stars for water. The reddest sources exhibit the lowest detection rate. It is found that the strong IR color dependence of water maser detectability depends primarily on excitation conditoins for the masers, rather than on a decrease in detectability with distance. This result can be explained by collisional excitation models for water masers, which feature an inner quenching radius. These models can also explain the variation in the velocity of water emission from OH39.7 + 1.5 with pulsational phase, which is seemingly a counterexample to collisional excitation models, as being due to a temperature modulation of the masing zone imposed by cyclical luminosity changes, and its effect on competitive gain between different ray paths.