Differential equations for the transmittance and the diffuse reflection coefficients of the stratified sea are obtained. To simplify starting radiative transfer equation, the approach uses experimental dependencies between mean cosines of underwater angular radiance distribution. The resulting equation for the diffuse reflectance coefficient is of a Riccati type. For the homogenous sea with arbitrary combination of inherent optical properties, that equation is solved analytically. For vertically inhomogeneous sea it is solved approximately. The resulting formula expresses diffuse reflection coefficient through the profiles of inherent optical properties of seawater and bottom depth and albedo. The results of calculations with main formula are compared with the Mont Carlo computations. It was found that the precision of this theory is about 15% and it is comparable with the precision of contemporary in situ measurements.