Abstract: An underwater communication system provides reliable underwater transmission of digitally encoded communications as a series of consecutive acoustic frequency pulses (40) wherein the frequency of each pulse corresponds to one of two preselected frequency settings (F.sub.A,F.sub.B). A communication is transmitted as a series of frequency pulses (40) divided into a header section (44), a message section (45), and a parity bit (46). The header bits are of a first pulse width (T1), and the message and parity bits are of a second pulse width (T2). Received communications are converted into an electrical receive signal by a hydrophone (76), which is processed by a sonar control (61) and provided to a CPU (62) which decodes the communication for display on a display unit (60).