Electrolyzing seawater to produce hydrogen can not only address the issue of freshwater scarcity but also provide an abundant raw material for hydrogen production. However, seawater electrolysis for hydrogen production still faces numerous risks and challenges at present. This study focuses on a systematic investigation of FeNiCo-based high-entropy alloy (HEA) nanocatalysts supported on carbon skeletons. By precisely regulating the morphological structure of the carbon skeleton, a carbon support with a large specific surface area and abundant active sites can be obtained. Simultaneously, the elemental composition of the HEA nanoparticles is adjusted to optimize its seawater electrolysis performance. An energy-saving strategy of coupling the anode sulfur oxidation reaction (SOR) with the cathode hydrogen evolution reaction (HER) is employed to assist seawater electrolysis. In alkaline seawater, at a current density of 10 mA·cm⁻², the overpotential of the HER is only 22 mV, and the overpotential of the oxygen evolution reaction (OER) is 264 mV. It also exhibits excellent performance in acidic seawater. In a two-electrode seawater electrolysis system, an applied voltage of 1.55 V is required to reach a current density of 10 mA·cm⁻². More importantly, when using SOR to assist alkaline seawater electrolysis, the applied voltage is successfully reduced to 0.82 V.