Free energies and dynamics of electron-transfer reactions for a diatomic donor-acceptor complex in ambient and supercritical water are studied via molecular dynamics computer simulations using a two-state electronic description. The free energy perturbation method is employed to examine diabatic electronic curves relevant to charge separation and recombination and electron self-exchange. It is found that the diabatic curves are anharmonic and vary with the charge distributions of the donor-acceptor complex, consonant with earlier Studies under ambient conditions. Nonetheless, the extent of their anharmonicity and dependence on charge distributions grows with decreasing solvent density so that the Marcus free energy relationship breaks down in low-density supercritical water. The influence of solvent dynamics associated with activation, deactivation, and adiabatic barrier crossing on reaction kinetics is analyzed. Although the transition state theory generally provides a reasonable framework to describe electron-transfer kinetics in a wide range of thermodynamic conditions, the deviation of the reaction rate from the transition state theory predictions increases as the water density decreases.