A new method of constructing an optical transfer operator for an active opto–electronic system of observation through a randomly rough interface in the "atmosphere––ocean" system is proposed. Based on the methods of the Green's functions and perturbation theory the general boundary–value problem is decomposed into a number of the simplest problems whose solutions are sought in the small–angle approximation of the radiative transfer equation. The expressions obtained take into account a radiation correlation on the randomly rough surface. They are compared with the results obtained by various authors.