Dynamic models of M-type Mira variables predict the occurrence of water \"shells\", i.e. of zones of high H2O density and high H2O absorption inside the stellar atmosphere. The density, position and width of these shells is closely correlated with different types of two-component shapes of the intensity distribution on the disk in the H, K and L near-continuum bandpasses. We investigate these correlations and highlight the role of a spectrophotometric H2O index that warns against serious complications in diameter measurements in the case of substantial water contamination of the bandpass of observation. Simultaneous spectrophotometric and interferometric measurements may allow observers to estimate real continuum diameters more precisely.