Time reversal (TR) achieves spatial and temporal focusing in complex environments. In the context of communications, passive (uplink) TR is equivalent to active (downlink) TR with the communications link being in opposite directions but with the same theoretical performance. The benefit of passive TR, however, is its natural self-synchronization, making TR communications robust and readily extended to cases involving temporal diversity and synthetic aperture communications. Self-synchronization is examined analytically and demonstrated with shallow water experimental data. In addition, the impact of spatial diversity on communications performance is investigated in terms of the aperture and element spacing of the 64-element vertical receive array.