The mechanism of electrons tunneling through a narrow water barrier between two Pt(100) metal surfaces is studied. Assuming an adiabatic picture in which the water configuration is static on the time scale of the electron motion, the tunneling probabilities are found to increase nonmonotonically as a function of incident electron energy. A numerical investigation of single electron scattering wave functions suggests that the tunneling is enhanced by resonances, associated with molecular cavities in which the electron is trapped between repulsive oxygen cores. The lifetimes of these resonances are calculated using a novel filter diagonalization scheme, based on a converging high-order perturbative expansion of the single-electron Green's function, and are found to be of order less than or equal to 10 fs. The possibility that transient resonance supporting structures contribute to the enhancement of tunneling through water is discussed.