Spectral measurements of downwelling and upwelling daylight were made in waters different with respect to turbidity and pigment content and from these data the spectral values of the reflectance ratio just below the sea surface, R(λ), were calculated. The experimental results are interpreted by comparison with the theoretical R(λ) values computed from the absorption and back‐scattering coefficients. The importance of molecular scattering in the light back‐scattering process is emphasized. The R(λ) values observed for blue waters are in full agreement with computed values in which new and realistic values of the absorption coefficient for pure water are used and presented. For the various green waters, the chlorophyll concentrations and the scattering coefficients, as measured, are used in computations which account for the observed R(λ) values. The inverse process, i.e. to infer the content of the water from R(λ) measurements at selected wavelengths, is discussed in view of remote sensing applications.