Rechargeable seawater batteries (SWBs) use Na+ ions dissolved in water (seawater or salt-water) as the cathode material. They are attracting attention for marine applications such as light buoys, marine drones, auxiliary power for sailing boats and so on. So far, SWB design has been developed from the coin-type to prismatic-shape cell for research purposes to investigate cell components and electrochemical behaviors. However, for commercial applications, that generally require >12 V and >15 W, the development of an SWB module is required, including cell assembly and packing design. The purpose of this work was to conduct research on the SWB cell assembly method while considering the SWB's properties and minimizing current imbalance. Additionally, a 5 Series (S) 4 Parallel (P) SWB module is constructed and validated using commercially available light buoys (12 V, 15 W).