We consider the shape distortions of narrow—band pulse signals during their propagation in an underwater acoustic channel. It is shown that this distortion is mainly determined by appearance of additional contribution to quadrature components. The contribution amplitude is connected with both a shape of emitted signal and channel parameters, namely, frequency dependence of attenuation. Additional contribution appearing in an underwater acoustic channel due to dispersion is detectable practically for all frequency ranges. The estimations of distortion value are presented for sea water as a function of distance and frequency.