Time synchronization is one of the most fundamental and widely employed services in networks. Because of long propagation delay in underwater acoustic sensor networks (UASNs), the time synchronization becomes more challenging. To solve these challenges, this paper presents a new synchronization and localization protocol (TSLA), which uses anchor nodes to broadcast one-way packets to synchronize and localize sensor nodes. Sensor nodes are in duty-cycle mode and only need to receive packets, which makes the algorithm energy efficient. Theoretical analysis and simulation results demonstrate the energy efficiency and high precision of TSLA.