The major problem encountered on underwater acoustic channels is that the system bandwidth is limited, while time-varying multipath propagation causes extreme signal distortion. Consequently, the data rate at which one can transmit through these channels is severely limited. In addition, acoustic signals propagate through the water at a very low speed (1500 m/s), and suffer from attenuation that depends not only on the distance, but also on the signal frequency. Together, these facts result in a communication medium that combines the worst of radio worlds: poor link quality of a terrestrial system, and high latency of a satellite system. As a result, acoustic communication system design poses many challenging problems.