Catalysis of the reaction: 2 H2O = O2 + 4 H+ + 4 e-. The evolution of oxygen from oxidizing water is carried out by the oxygen evolving complex in photosystem II of plants. P680+, the photochemically oxidized reaction-center chlorophyll of PSII, is a strong biological oxidant. The reduction potential of P680+ is more positive than that of water, and thus it can oxidize water to give O2 and H+ ions. The oxygen escapes as a gas while the H+ ions remain in solution inside the thylakoid vesicle.