The search for efficient oxygen evolution reaction (OER) electrocatalysts capable of high-current-density water electrolysis is critical for scalable hydrogen production. Herein, we present a rationally designed FeCo-LDH@Ni₃S₂ heterostructure on nickel foam (NF), synthesized through a controlled approach. This electrode delivers ultralow overpotentials of 220, 235, and 245 mV at 10 mA·cm⁻² in alkaline freshwater, simulated seawater, and natural seawater, respectively, alongside remarkable 100 h stability at industrial-level conditions (100 mA·cm⁻² in seawater). Furthermore, a symmetric electrolyzer utilizing FeCo-LDH@Ni₃S₂ as both cathode and anode achieves low voltages of 1.60, 1.64, and 1.69 V at 10 mA·cm⁻² in the corresponding electrolytes and exhibits over 100 h stability at 50 mA·cm⁻². Density-functional theory (DFT) analysis confirms that the FeCo-LDH@Ni₃S₂ heterointerface enables charge redistribution, optimizes the d-band center, and reduces the energy barrier for OER rate-determining steps. This study demonstrates an effective interface engineering strategy for d-band center reduction via heterostructure design, offering a durable electrocatalyst for marine hydrogen production.