Renewable-powered seawater electrolysis for green hydrogen is hindered by chloride corrosion. We introduce a protective interfacial water film strategy, where a highly ordered hydrogen-bonded network in the outer Helmholtz plane (OHP) leverages strong covalent O–H bonds and dense ice-like ordering to block Cl− ingress. This engineered barrier within the electric double layer prevents chloride access to the inner Helmholtz plane (IHP), eliminating corrosive complexation. The film is realized via dynamic phosphorus migration during the electrochemical reconfiguration of a phosphate-doped cobalt–nickel–iron layered hydroxide (CNFPO), enriching phosphate species that electrostatically template interfacial water molecules. Combined in situ Raman spectroscopy (using D2O) and molecular dynamics reveal the enrichment of tetrahedral hydrogen-bonded water in the OHP, forming the Cl−-repelling barrier while the participation of free water in the reaction pathway is further verified via H2O/D2O kinetic isotope effect (KIE) experiments. Protected by this film, CNFPO achieves an overpotential of 24 mV at 10 mA cm−2, >1200 h stability, and 76-fold current density enhancement in saturated saline. An anion-exchange membrane electrolyzer operates for >1000 h at 500 mA cm−2 and 60 °C, validating practical viability.