As in all distributed systems, time synchronization plays a significant role in underwater sensor networks. Synchronization of clock values among the contributing nodes is one of the crucial prerequisites in addressing network related issues of Underwater Wireless Sensor Networks (UWSNs). The traditional terrestrial sensor network technologies like, the Global Positioning System (GPS) cannot be utilized proficiently for underwater scenarios due to its bandwidth limitations, channel complexities and network cost. All these together make the synchronization problem extremely challenging for UWSNs. Synchronization techniques assisted by a mobile beacon situated either on the ocean surface or along a predefined trajectory are intrinsically convenient, accurate at a reduced cost and energy supplies. In this paper, we propose a novel underwater synchronization approach based on directional beacons, called DB-Sync. The proposed approach employs directional signals, transmitted by an Autonomous Underwater Vehicle (AUV). The proposed scheme does not require the mutual communication between the AUV and the sensor nodes, thereby reducing the transmission energy consumption which is a non-trivial factor of sensor networks. The proposed scheme is analyzed using simulations and the results prove that our DB-Sync outperforms existing approaches in a precise, energy efficient fashion. © BEIESP.