An inelastic neutron scattering experiment has been performed on supercritical water at high momentum transfer, up to 90 Angstrom (-1), in order to study single proton dynamics. The value of the proton mean kinetic energy has been extracted in the framework of the impulse approximation. The anisotropy of the proton momentum distribution inside a single water molecule is discussed. The extracted experimental mean kinetic energy is found in good agreement with the predictions of a harmonic model, under the assumptions of decoupling between translational, rotational and vibrational degrees of freedom. Differences emerge between our results and those obtained in a recent inelastic neutron scattering experiment on water in sub-and supercritical conditions. These differences are pointed out and examined in detail.