A two-dimensional multiple-histogram method for isothermal-isobaric ensemble is discussed in detail, implemented for isothermal-isobaric Monte Carlo simulations of molecular clusters, and employed in a case study on phase changes in pure water clusters containing 15 through 21 water molecules. Full phase diagrams of these clusters are reported in the temperature-pressure plane over a broad range of temperatures (T = 30-800 K) and pressures P = 10(3)-10(9) Pa. The main focus of the work is on the structural transformation occurring in the solid phase of these clusters and leading from cluster structures with all molecules on the cluster surface to cage-like structures with one molecule inside, and on how the transformation is influenced by increased pressure and temperature