Abstract--Time synchronization is a critical service for distributed network systems. In this work, we investigate this problem in the context of underwater sensor networks (UWSNs). Although there are many time synchronization protocols proposed for terrestrial wireless sensor networks, none of them could be directly applied to UWSNs. This is because most of these protocols do not consider long propagation delays and sensor node mobility, which are important characteristics in UWSNs. Further, UWSNs usually have very high requirements in network lifetime and synchronization accuracy. To satisfy these needs, innovative time synchronization solutions are demanded. In this paper, we propose a novel time synchronization scheme, called "Mobi-Sync", for mobile underwater acoustic sensor networks. Mobi-Sync novelly utilizes the spatial correlation of underwater mobile sensor nodes to estimate the long dynamic propagation delays. Simulation results show that Mobi-Sync outperforms exis...