A covert underwater acoustic waveform is presented. The modulation scheme is based on a pseudo-random selection of spread-spectrum sequences from a dictionary. A specific aspect of this scheme is that each sequence has a distinct chip rate. The use of spreading sequences is a means of operating at low SNR, while the use of different chip rates strongly attenuates the cyclostationary signature, the objective being to maximize the robustness against eavesdropping attacks. Simulation results and sea trials show that communication can be successful at negative SNRs while being robust to cyclostationary analysis.