Photovoltaic hydrogen production from seawater is of great significance. Challenges of solar-driven seawater electrolysis, for example, competing among chlorine evolution reactions, chloride corrosion, and catalyst poisoning, seriously restrict the development of this technology. In this paper, we report a two-dimensional nanosheet quaternary metal hydroxide catalyst composed of Ni, Fe, Cr, and Mo elements. By in situ electrochemical activation, a partial Mo element was leached and morphologically transformed in the catalyst. The higher metal valence states and many O vacancies were obtained, providing excellent catalytic activity and corrosion resistance in overall alkaline seawater electrolysis operating at an industrial-required current density of 500 mA cm over 1000 h under 1.82 V low voltages at room temperature. The floating solar seawater splitting device shows a 20.61 ± 0.77% efficiency of solar energy to hydrogen (STH). This work demonstrates the development of efficient solar seawater electrolysis devices and potentially promotes research on clean energy conversion.