Juniper Publishers - Journal of Oceanography Abstract Today, underwater acoustic communication is developing for its various applications. For signal transmission in underwater using ocean/ sea/lakes/rivers as a medium an appropriate underwater acoustic system is required. The significant characteristic of ocean makes the underwater communication difficult using electromagnetic waves. The characteristics of an acoustic signal make a good candidate for data transfer in underwater. The underwater communication is furthermore challenging due to factors like reverberation, attenuation, scattering and sea characteristics, etc., apart from various sources of ambient noises due to natural and man made. Thus, an underwater acoustic modem plays a vital role for long distance signal transmission in underwater. Hence, it is important to study the various types of underwater acoustic modems. This paper provides a comprehensive overview of recent development and challenges in var
