The current problem of classifying marine mobile objects according to hydroacoustic monitoring data is considered. The main focus is on the analysis of the low-frequency part of the spectrum, where key identification information is concentrated due to propeller noise and mode frequencies from ship noise in shallow water. A new method of harmonic periodograms is proposed, which makes it possible to identify and analyze harmonic structures in noisy marine environments. Emphasis is placed on the problem of reliably determining the basis of harmonicseries of a hydroacoustic marine mobile object with a low signal-to-noise ratio and the presence of non-stationary interference. The results of experiments on full-scale sonar recordings are presented, demonstrating the effectiveness of the proposed approach. It is shown that the use of features based on harmonic series, combined with the identification of discrete components of a shallow-water channel, makes it possible to achieve high accuracy in classifying various types of vessels and their motion parameters. It is concluded that the development of this area is promising for the creation of automated situational awareness systems in the marine environment.