Collisional energy transfer between two water molecules, one highly energized (reactant) and another thermally equilibrated (medium) molecule, has been studied by classical molecular dynamics simulation over a range of excitation energies and medium temperatures. The focus is on the dependence of the energy transfer efficiency on the excitation energy, the medium temperature, and the gross features as well as the details of the interaction between the molecules. High quality interaction potentials based on experimental data or quantum chemical calculations are used and the results are compared with those obtained by simpler potentials constructed from Lennard-Jones pair potentials and point charges. The dipolar contribution to the interaction is varied and the molecules are partially or fully deuterated. The strong electrostatic interaction is found to yield efficient energy transfer for small impact parameters but also a large cross section for water collisions. The energy transfer efficiency is sensitive to the detailed form of the interaction. However, if somewhat lower accuracy can be accepted then simple potentials can be used. The energy transfer can be well fitted by a conditional probability density based on a statistical model of equilibration among subsets of the degrees of freedom in the colliding molecules. Rotational energy transfer is far more efficient than vibrational energy transfer.