an underwater apparatus for transmitting and receiving high rate data and voice communication including a transmitter , a receiver , and a doppler frequency shift compensator . the transmitter includes a data source comprising digital data to be transmitted by the apparatus through the water , a serial - to - parallel data processor for splitting the serial digital data into n parallel data channels , a n - channel modulator for receiving the n parallel data channels and for modulating those channels with n pairs of ultrasonic carriers to produce a modulated signal . the transmitter also includes a hydrophone for receiving and transmitting the modulated signal . the receiver includes a hydrophone for receiving a modulated signal . the receiver also includes rf circuitry for amplifying and shaping the received modulated signal , a serial - to - parallel data processor for splitting the amplified and shaped data into n parallel channels , a n - channel demodulator for demodulating the shaped signal and for outputting n channels of digital data , and a parallel - to - serial data processor for receiving n parallel channels of data from the demodulator and for combining those channels into serial data . the doppler compensator measures the frequency of at least one of two unmodulated signals transmitted as part of the modulated signal and compares the measured frequency with a predetermined frequency . operation of the demodulator is adjusted to compensate for any deviation between the measured and predetermined frequency .