Results of VLA observations of 22 GHz H2O masers associated with 15 Mira and semiregular variables are reported. The combination' of angular resolution (almost-equal-to 70 mas), spectral resolution (0.3 km s-1), rms sensitivity (+/-35 mJy beam-1), and hour-angle coverage is the best yet obtained for H2O Masers from a significant sample of stars. Comparison of accurate optical positions of the stars with estimates from the masers yields total differences typically less-than-or-equal-to 0.15''-an improvement over previous comparisons by a factor of about 2. The homogeneous, high-quality data set provides several new insights into the time-varying structure of circumstellar H2O profiles and angular distributions. Profile structures can exhibit dramatic, short-term (less than 1 yr) changes but also may exhibit stability of features over timescales up to 15 yr after allowance for phase-dependent effects. For this sample of stars (rates of mass loss from about 10(-7) to 10(-6) M. yr-1), the average profile shape shows strongest emission generally within +/-2 km s-1 of the stellar velocity V0, regardless of the stellar light-curve phase phi, but there is a higher probability of detecting multiple (particularly blueshifted) features and a larger velocity range (V0 +/- 9 km s-1) at 0.2 less-than-or-equal-to phi less-than-or-equal-to 0.4, when the integrated H2O luminosity is largest. Based on observations of different stars and on repeated VLA observations of specific stars, the shell size may vary typically by about a factor less-than-or-equal-to 2 during the light cycle, with statistically largest radius [rho] almost-equal-to 25 AU) at 0.2 less-than-or-equal-to phi less-than-or-equal-to 0.4. The morphology of the angular distribution can change strongly with phi, but similar morphologies are sometimes observed at comparable phases of different light curves, indicating that short-term changes (less than a few years) are likely to reflect changes in the pumping conditions. The shell radii range from greater-than-or-equal-to 5 to about 50 AU, and the region of maximal H2O intensity at a given epoch appears to occur over a radial interval less than or equal to one-third of the shell radius. The angular distributions at a given epoch frequently are elongated and sometimes are strongly asymmetric relative to the estimated stellar position. For three stars (RT Vir, U Her, UX Cyg), thin, asymmetric loop structures are found. For U Ori, strongest components near V0 are confined northeast and southwest of the star, suggestive of axial symmetry. Comparison of OH and H2O distributions shows points of similarity for several stars. Plots of shell radius as a function of radial velocity exhibit two forms of curves: singly peaked curves, where the radius increases rapidly within a few kilometers per second of V0, and doubly peaked curves, where the maximum radius occurs at velocities symmetric to but displaced from V0. The latter type of curve also tends to be double-valued (X-shaped) with two distinct map maxima at a given velocity over significant velocity intervals. There are indications of some combination of nonuniformities in the density distribution, deviations from sphericity, and anisotropies in the velocity field, but it is difficult to disentangle the effects of these phenomena and achieve unique interpretation. A plausible model is clumps or filaments distributed at radii which vary with direction from the star. From the angular distributions and velocity ranges, there is good evidence that at least some parcels of gas are accelerated to the terminal outflow velocity at rho greater-than-or-equal-to 20 AU and that there is a component of outflow at rho almost-equal-to 10-15 AU; some gas probably is fully accelerated at radii less than 10 AU. To illustrate the problems of interpretation, the well-known case of VX Sgr is reexamined. The data do not support a simple model of increasing outflow velocity with increasing maser shell radius. It is proposed that the outflow is weakly bipolar.
