Harvesting energy from natural resources is of significant interest because of their abundance and sustainability. In particular, large-scale marine energy storage shows promising prospects because of the massive and diverse energy forms such as waves, tide and currents; however it is greatly hindered due to its complicated circumstances and intermittent nature. Storing and transporting locally generated energy has become a vital step for future sustainable energy supplies. Here, we proposed a highly-extensible “paper-like” all-in-one seawater supercapacitor constructed from a nanofiber-based film in operando towards electrochemical energy storage in the marine environment, which features lightweight and excellent mechanical properties with a typical thickness of about 100 μm. The single supercapacitor cell shows a remarkable performance with an energy density of 6.6 mW h cm−3 at a power density of 99.0 mW cm−3, and exhibits a capacitance retention of 100% under different bending operations. Moreover, the large-scale extensibility of the all-in-one seawater supercapacitor cell was fully demonstrated with an optimized circuit design. The integrated device connected with multiple cells in series and parallel can successfully drive a motor with a voltage of 12 V and a power of 2.5 W for operation. It shows prospective applications for future large-scale distributed energy storage systems in the marine environment.