Time synchronization is the basis for collaboration across multiple sensor nodes.Underwater sensor networks (UWSNs) have different characteristics from the terrestrial wireless sensor networks

where communication is primarily via acoustic signals

producing new challenges to time synchronization.This paper first illustrates the definitions of the problem and algorithm of time synchronization

describes the major differences between underwater acoustic communication and terrestrial wireless communication

and discusses the influences to time synchronization causing by these differences;then reviews the terrestrial time synchronization algorithms

pointing out the reasons why such algorithms cannot be applied to underwater environment.Afterwards

a thorough review of the research progress on time synchronization for UWSNs has been presented with the performance comparison among four major algorithms through simulations.Finally

we summarize the crucial issues on time synchronization for UWSNs

and indicate the future research directions.