A method for mobile underwater acoustic communications includes double differentially (DD) encoding a communication signal to produce a DD-encoded communication signal, applying direct sequence spread spectrum (SS) to the DD-encoded signal to produce a DD-SS communication output signal, and transmitting the DD-SS communication output signal. The method i) increases the SNR via processing gain, ii) eliminates the ISI through multipath suppression, and iii) enables bandwidth efficiency improvement via data multiplexing. The method is shown capable of facilitating simple receiver processing and offering performance robustness against unpredictable channel fluctuations.