Seawater electrolysis offers a sustainable route for large-scale hydrogen production, yet the development of nonnoble metal electrocatalysts with high activity and chloride resistance remains challenging. Here, we report a photo-assisted synthesis method to fabricate a ternary metal (oxy)hydroxide electrocatalyst (p-S-Ni(Fe,Co)OOH) from nickel foam, utilizing photogenerated holes to achieve atomic-level Fe/Co co-doping. The p-S-Ni(Fe,Co) OOH exhibits bifunctional activity in alkaline seawater, requiring overpotentials of 108 and 186 mV for hydrogen- and oxygen-evolution reactions (HER and OER) at 10 mA cm- 2 in 1.0 M KOH, respectively, outperforming its dark-synthesized counterpart (S-Ni(Fe,Co)OOH). When integrated into a photovoltaicelectrocatalytic (PV-EC) powered by commercial Si solar cells, p-S-Ni(Fe,Co)OOH achieves solar-to-hydrogen (STH) efficiencies of 13.2 % (simulated seawater) and 12.1 % (natural seawater), with stable operation over 30 h. Density functional theory (DFT) calculations identify Fe as the primary active site for OER, while Fe/Co codoping modulates the Fe d-band center toward the Fermi level, optimizing intermediate adsorption energetics and improving chloride resistance. This study proposes a synthesis strategy for seawater-compatible catalysts elucidates electronic structure modulation at colloidal interface.