A multiple-access underwater acoustic cellular network is considered in this paper using direct sequence spread spectrum techniques similar to the code division multiple access cellular network in RF communications. To keep a reasonable data rate, in view of the limited bandwidth in an underwater (UW) channel, the length and, therefore, the number of orthogonal codes (the number of users) cannot be too large. At the same time, the orthogonality of the codes is severely degraded by the extended multipath arrivals in the UW channel. As a result, communication bit error rate becomes non-negligible when the interferer's signal energy (due to increasing number of users) becomes order of magnitudes higher than the desired signal [Yang et al. JASA 126, 220–228 (2009)]. In this paper, techniques used in multiple-input-multiple-output communications are applied to underwater cellular network for simultaneous communications between the users and the base station, taking advantages of the (rich) spatial degrees of freedom of the UW channel. Simulation and experimental results will be presented. [Work supported by the U.S. Office of Naval Research.]