To solve the problem of low bandwidth efficiency of underwater acoustic spread spectrum communication, a multicarrier modulation scheme combined with M-ary spread spectrum and cycle shift keying (CSK) is proposed in this paper. The code sequences chosen by M-ary modulation are then modulated by CSK, simultaneously, they are modulated by the orthogonal multicarrier. The approach enhances the bandwidth efficiency greatly. The performances of the system with m-sequence, Kasami-sequence, and Gold sequence as the code sequence are compared over the underwater acoustic multipath channel with the same data rate and the same bandwidth. The simulations and the results of pool experiment show that m-sequence as the code sequence performs best and Gold sequence performs worst. The number of the m-sequence is limited. In order to increase the data rate further and make the performances of other sequences close to the m-sequence, a new approach to suppressing the inter-carrier interference (ICI), named multiple sequences, is proposed. The approach can greatly suppress the ICI caused by multipath and make the performance close to the performance of the m-sequence.