We have carried out path integral- molecular dynamics simulations to describe microscopic details of excess electrons in supercritical water over a wide range of solvent densities, rho(w), along the T = 645 K isotherm. The well-tested simple-point charge model for water was used. The transition from localized to quasifree states described in terms of the electron spatial extent is observed in, the vicinity of rho(w) = 0.15 g cm(-3). For smaller densities, the electron undergoes quantum tunneling through nearest neighboring water molecules. The ground-state absorption spectrum exhibits significant red shifts in the absorption maxima with decreasing density, showing reasonable agreement with recent pulse radiolysis measurements.