Time synchronization and localization issues in underwater sensor networks, where more challenges are introduced due to the unique properties of the water environment. These challenges include long duration and transmission delay, low bandwidth, energy restriction, mobility, etc., we recommend the time synchronization and localization based on the semi-periodic nature of seawater movement, SLSMP called. First, we analyze error factors in time synchronization and localization, and then propose a method to deal with this error. For a more precise synchronization, SLSMP controls the time of transmission by the pattern of movement and seawater node providing exploits. Then SLSMP gradually decreases averaged localization error by the Kalman filter application or filter. Finally, INS (Inertial Navigation System) was adopted by the node mobility and error propagation problem caused localization error relieve. The simulation results show that SLSMP time synchronization error of 2.5 ms and 0.56 ms compared to TSHL and MU-Sync or reduced. Also, localization error of 44.73% reduced compared to the single multilateration system.