The book discusses the main features of the propagation of sound waves in the oceans and seas, the principle of operation and arrangement of acoustic antennas and other elements of hydroacoustic equipment. The reader will learn about the principles of operation and operation of the most important sonar instruments, widely used in solving various tasks related to human activity in the depths of the sea: sonar, noise-peeling devices, communication devices, fish-search devices, navigation equipment .. The book is designed for a wide range of readers: radio amateurs and all interested in the current state of various types of electronic equipment