An environmental-friendly supercapacitor based on aqueous electrolyte was fabricated. Electrodes with conductive spider nest–shaped three-dimensional (3D) porous structures were prepared for the assembly of symmetric supercapacitors. The nickel foam was modified by multiwalled carbon nanotubes and β-cyclodextrin. The construction of the spider nest was stabilized via the chemical bond inside carbon nanotubes, π–π stack effects among carbon nanotubes, and physical adsorption between nickel foam and carbon nanotubes substrate. The role of β-cyclodextrin is a dispersant to prevent agglomeration of carbon nanotubes, thereby enhancing electroactive surface area of nickel foam and improving the specific capacitance of the electrodes. Furthermore, the electrodes exhibited excellent rate capability. The obtained symmetrical supercapacitors exhibited an excellent power density of 17,561.3 W kg−1, a good specific capacitance of 398.8 F g−1, and an energy density of 154.8 Wh kg−1 for 4000 cycles with outstanding cycling stability. In addition, the specific capacitance, energy density, and power density of the supercapacitor operating in seawater were found to be 100.2 F g−1, 17.8 Wh kg−1, and 2568 Wh kg−1, respectively, for 3000 cycles. Overall, our findings indicate that the supercapacitor could stably operate in seawater and shows potential for use as an ecofriendly power supply to marine engineering equipment.