The importance of accurately accounting for all Coulombic forces in molecular dynamics simulations of water at interfaces is demonstrated by comparing the Ewald summation technique with various spherical truncation methods, The increased structure induced by truncation methods at 12 Angstrom leads to water/vapor surface tensions and surface potentials that are respectively 50% and 100% greater than obtained with Ewald. The orientational polarization of water at the lipid/water interface is analyzed within the Marcelja-Radic theory of the hydration force, yielding decay parameters of 2.6 and 1.8 Angstrom for spherical truncation and Ewald, respectively, as compared with 1.7-2.1 Angstrom obtained from experiment. Bulk water transport properties such as the viscosity and diffusion constants differ by as much as 100% between simulations carried out with and without truncation; this may be related to ordering in the neighborhood of the cutoff radius. The diffusion constant calculated from the Ewald simulation is significantly further from experiment than the cutoff result, pointing out the need to reparametrize the TIP3P water model for use with Ewald summation. Appendices describe a method for carrying out the Ewald summation on a distributed memory parallel computer and other computational details relevant when simulating large systems.