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Owing to multiple access capability, low probability of intercept and immunity against inter symbol interference (ISI) caused by multipath propagation, direct sequence spread spectrum (DSSS) techniques have been utilised in both underwater acoustic (UWA) communication networks and reliable point-to-point communications. To overcome the severe impact of ISI in the UWA channels, we propose a novel multiple pseudo noise (PN) sequence DSSS scheme and analyse its performance in sparse time varying UWA channels. Analytical and simulation results show that using a few PN sequences can remove most of the multipath induced ISI. To further alleviate the ISI effect, we propose a simple ISI reduction algorithm. Numerical results show that our proposed algorithm along with the proposed DSSS scheme can effectively mitigates the effect of remaining ISI, especially in sparse multipath channels.