The development of cost-effective, durable bifunctional catalysts is pivotal for scalable green hydrogen production. Herein, a CuCo2S4@Ni(OH)2 heterostructure was fabricated via a hydrothermal method, delivering remarkable catalytic activity towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both alkaline and seawater media. The strong electronic coupling at the CuCo2S4@Ni(OH)2 interface enhances charge transfer and optimizes intermediate adsorption, enabling efficient water splitting. Benefiting from these features, the CuCo2S4@Ni(OH)2 achieves overpotentials of 134.3 mV and 187.6 mV at 10 mA cm-2 for HER and OER, respectively. A CuCo2S4@Ni(OH)2 electrolyzer delivers a cell voltage of 1.52 V at 10 mA cm-2 in KOH and excellent seawater activity with low overpotentials (124.1 mV for HER, 186.4 mV for OER) and minimal degradation. Density functional theory analysis reveals that charge migration from Ni(OH)2 to CuCo2S4 enhances catalyst's conductivity. This study demonstrates sulfide-hydroxide heterostructures as efficient, stable catalysts for large-scale hydrogen generation.