The influence of the illumination by the direct sun light and the diffuse light of the sky on the spectral structure of the apparent optical properties of seawater is studied. The resulting formula of this paper couples the sea diffuse reflection coefficient with the angular distribution of the natural light and the inherent optical properties of the seawater. This work uses a self-consistent approach to solve the radiative transfer equation. That approach was developed earlier to calculate the apparent optical properties including the diffuse reflection coefficient. A model of the inherent optical properties of seawater is proposed. This model expresses the inherent optical properties through the concentrations of chlorophyll, yellow substance, and biogenic and terrigenic hydrosols. The transformations of sea optical signatures, due to the changes in illumination and concentrations of suspended and dissolved matter, are analyzed. It is shown that the atmospheric optical parameters and the sun elevation angle significantly influence optical signatures of the upwelling light. The effective wavelength -- the parameter that is weakly dependent on the conditions of illumination -- is proposed.