Abstract: This paper presents the idea and methods of approximation of underwater channel impulse response by non-orthogonal functions. The family of functions that correctly estimate response of underwater channel is chosen. For these functions, method of calculation of parameters and coefficients is introduced. Besides that, two examples are presented. The first one is the approximation of response in deep reservoir, the second one is approximation of response in shallow reservoir, where appear problems of reflections from surface of water and bottom of reservoir