The quantum energy gap law for electron transfer (ET) reactions in water is examined. Molecular dynamics (MD) simulation analysis is carried out to obtain the solvent reorganization energies, time correlation functions (TCF), spectral density functions, and quantum rate constants. Their dependence on the reaction free energy and on the donor-acceptor distance is explored along with the solvent isotope effects. Properties of the imaginary-time saddle-point for the TCF expression of the ET rate formula are also examined. The high-frequency intramolecular vibrational modes of the solvent water are found to present marked quantum effects on the ET rate, while their contribution to the static reorganization energy is small (less than 6%). The energy gap dependence of the quantum activation free energy is shown to become nearly independent of the donor-acceptor distance when renormalized by the reorganization energy. Approximations to compute quantum rate constants from MD simulation data are briefly discussed in light of of the present results.