A series of molecular dynamics (MD) simulations has been performed to investigate the water/platinum interface. Water laminae between Pt walls and water monolayers on platinum were simulated under the influence of an external electric field acting on the molecules. The system is thought to be a realistic model for a water/metal interface. From the computer simulations structural and dynamical properties of water are calculated and the double layer is characterized. The dependence of the water structure, especially the orientational structure, on the electrical field strength is investigated as a function of distance from the surface. Electrostatic potential profiles across the interface, work function changes, and dielectric properties of water are calculated. The double-layer capacitance is estimated and found to be almost independent of the external field. The dipolar relaxation mechanisms in the adsorbed layer and in the bulk phase are discussed on the basis of autocorrelation functions of the dipole moment vector calculated for various field strengths. The spectral densities of librations and internal vibrations are only slightly influenced by the application of an external electric field.