Earth-abundant, non-toxic, and stable porous Co3O4 film grown by Kirkendall-diffusion is developed for efficient photoelectrocatalytic seawater splitting. Semitransparent Co3O4 films are found highly active hydrogen evolution reaction (HER) photocatalyst with stable performances. Dual band gap (1.5 eV and 2.3 eV) p-type Co3O4 samples exhibiting photo induced HER current density of 25 mA/cm(2) is obtained with overpotential of 0.83 V in the sea-water. Our Co3O4 sample possesses Tafel slope of 20 mV/dec, incident photon to the current conversion efficiency of > 8%, and charge transfer resistance of 3.4 Omega/cm(2). Photo induced Mott-Schottky analysis was applied to find the flat band potential corresponding to the lower band gap value and exhibited strong influence on the majority carrier concentrations and shifting of flat band potential. Electrical properties of Co3O4 material and losses associated in seawater splitting were carefully studied using the Mott-Schottky and electrochemical impedance spectroscopy. Present studies on seawater splitting using Co3O4 not only suggest photoelectrocatalytic hydrogen generation but also usefulness for efficient sea salts formation.