An underwater communication system of the present invention comprises a transmitter including message inputting means having keys, each being assigned to each word, code converting means for converting a key input entered through the inputting means to a code assigned to the key input, modulator means responsive to the code for performing modulation, and transmit transducer means for converting an output of the modulator means to an acoustic wave, and a receiver including a receiver transducer means for reconverting the incoming acoustic wave from the transmitter to an electric signal, demodulator means for demodulating the electric signal to generate the code, speech synthesis means responsive to the demodulated code for producing a word corresponding to the code as a speech, and speaker means for outputting the synthesized speech.