The operation of underwater acoustic communication systems is often directly or indirectly limited by the available temporal bandwidth with performance measures, such as range and information rate, closely tied to the system bandwidth efficiency. Spatial diversity has typically been viewed as a secondary channel feature, either introducing undesirable intersymbol interference or, more favorably, offering a mechanism to offset fading. It is proposed to treat spatial diversity as spatial bandwidth and demonstrate higher bandwidth efficiency with existing power and temporal bandwidth constraints. Several spatial modulation algorithms are explored, including time variant transmitter beamforming where both waveform and choice of projector weights denote information, a spatial analog to multi-carrier modulation where each propagation path corresponds to a carrier, and a coordinated multi-user scenario where each projector emits independent information. Performance of each algorithm is shown to be superior to conventional phase-coherent modulation techniques that make no explicit use of spatial bandwidth. Results of a small-scale field experiment will also be given.