Direct photoelectrochemical splitting of natural seawater offers a scalable route to green hydrogen but is constrained by an interfacial paradox: alkaline conditions that accelerate hydrogen evolution also induce Mg/Ca hydroxide scaling, rapidly blocking semiconductor surfaces. Here, we report a membrane-defined III–nitride photocathode that stabilizes the seawater interface and enables catalyst-free hydrogen evolution directly in Yellow Sea seawater. N-polar InGaN quantum pyramids integrated on vertical GaN nanowires provide broadband (UV–NIR) absorption and a composition-graded “quantum-sliding” band profile, funneling photogenerated electrons toward the pyramid apex and the catalytically active N-polar surface. A conformal graphene-oxide/polylactic-acid (GO/PLA) nano-membrane laminated on the nanowire forest serves as an ion-sieving transport layer, preserving water access while suppressing multivalent-ion-driven precipitation and electrode blockage. X-ray photoelectron spectroscopy quantifies heterointerfacial band offsets, while Raman spectroscopy and electron microscopy confirm conformal encapsulation and mitigated deposits after operation. The photocathode sustains ~0.5 mA cm−2 at −0.45 V versus RHE for over 200 h in untreated Yellow Sea seawater (pH 7) and reaches ~−16.51 mA cm−2 in Yellow Sea water containing 0.5 M NaOH, with onset potentials of 0.26–0.29 V versus RHE. This membrane–band co-engineering strategy decouples semiconductor photophysics from electrolyte complexity, enabling practical solar hydrogen production from unprocessed seawater.