Abstract: Modem for underwater communication is two-way communication hydroacoustic system. Modem uses frequency modulation FSK data in the aquatic environment. The paper describes the different blocks of the modem, which include system transmitter, receiver and processor chip. Describes the basic parameters of the modem, such as a modem coverage of 2000m, a carrier frequency of 35kHz. It also presents the results of laboratory tests carried out in an anechoic pool measuring and test results carried out in the real world for two bodies, a closed basin yacht marina, where there were reflections from the walls of the pool and the harbor channel, where the level of the reflected signal was negligible. In an environment where there were strong-multipath reflections, it was necessary to shorten the duration of the bit and increase the time between bits to get the correct data transmission. Describes the initialization procedure calls and commands sent between the individual above water, and stand-alone measuring module.