Transition metal phosphides (TMPs) are emerging as the indispensable electrocatalysts for overall water/seawater splitting, while there are ongoing challenges resulting from the poor catalytic activity and low stability, especially the electrode corrosion caused by the side reactions of chloride evolution reaction (ClER). Herein, Mo-decorated CoP_X nanoarrays on nickel foam (Mo-CoP_X/NF) is prepared by an in situ hydrothermal-phosphorylation method. Profiting from the unique self-supported nanoarrays, plenty of bimetallic active sites, and high conductivity, Mo-CoP_X/NF nanosheet arrays show low overpotential as bifunctional electrocatalyst in water/seawater electrolytes. The Mo-CoP_X/NF || Mo-CoP_X/NF pair only needs 1.49 and 2.01 V to drive an overall water splitting of 10 and 100 mA cm⁻², respectively, which is much better than the commercial catalysts. Moreover, with the strong inhibiting effect of ClER and corrosion resistance benefited from the synergistic effect of bimetallic atoms in phosphide, Mo-CoP_X/NF || Mo-CoP_X/NF pair has superb catalytic durability for seawater electrolysis.