<p>Photocatalytic seawater splitting provides a sustainable route to green hydrogen, but faces barriers of inefficiency and scalability. Graphitic carbon nitride (g-C₃N₄, CN) offers saline tolerance, yet its catalytic potential requires atomic-level control of structure and charge dynamics. Herein, we design three single-atom Co configurations on CN, including symmetric Co–N₄, vacancy-anchored Co–N₃, and asymmetric Co–N₄. The asymmetric Co–N₄ in hierarchically porous carbon nitride (CoSA-hCN) introduces second-shell carbon vacancies that reshape electronic asymmetry and charge dynamics, establishing a robust pathway for effective salinity-mediated charge transfer and in situ Pt photodeposition for H₂ evolution. On a scalable 60 cm² floating photothermal platform under 1 sun, H₂ production (47.7 mmol m⁻² h⁻¹) and interfacial seawater evaporation (1.88 kg m⁻² h⁻¹) are achieved directly from natural seawater. In this work, we offer design principles for a single-atom catalyst on scalable photothermal platform for coupled seawater splitting and desalination.</p>