This textbook on Underwater Acoustics has a structure that is more organic than logical. It thereby unifies diverse areas of research, including topics of signal processing, the sonar equation, sources and receivers, scattering and reverberation, wave propagation, propagation models, and inverse problems. It also provides code fragments written in Python which complement the discussion. This is a book written for both beginners and specialists, as well as for biologists, oceanographers, computer engineers, physicists, and mathematicians, and for civilian and naval personnel who are looking for a introductory overview of the topic.