The underwater acoustic channel exhibits large dispersion of the signals in time and frequency domains. The dispersion is characterized by broadening and fading of the signals transmitted through it due to multipath propagation and random motion of particles constituting the medium. Conventional modulation/demodulation techniques, even with forward error correction and ARQ techniques and at moderate data rates, are quite ineffective for data communication over this channel. This paper briefly reviews som signalling schemes which might be suitable for the underwater acoustic channel. A comparative evaluation of them is also presented. The paper also describes a microprocessor-based instrumentation of a versatile MODEM which can implement these schemes under program control with a fixed amount of associated hardware.