Designing nonprecious metal‐based electrocatalysts to yield sustainable hydrogen energy by large‐scale seawater electrolysis is challenging to global emissions of carbon neutrality and carbon peaking. Herein, a series of highly efficient, economical, and robust Ni–P‐based nanoballs grown on the flexible and anti‐corrosive hydrophobic asbestos (NiPx@HA) is synthesized by electroless plating at 25 °C toward alkaline simulated seawater splitting. On the basis of the strong chemical attachment between 2D layered substrate and nickel‐rich components, robust hexagonal Ni5P4 crystalline modification, and fast electron transfer capability, the overpotentials during hydrogen/oxygen evolution reaction (HER/OER) are 208 and 392 mV at 200 mA cm−2, and the chronopotentiometric measurement at 500 mA cm−2 lasts for over 40 days. Additionally, the versatile strategy is broadly profitable for industrial applications and enables multi‐elemental doping (iron/cobalt/molybdenum/boron/tungsten), flexible substrate employment (nickel foam/filter paper/hydrophilic cloth), and scalable synthesis (22 cm × 22 cm). Density functional theory (DFT) also reveals that the optimized performance is due to the fundamental effect of incorporating O‐source into Ni5P4. Therefore, this work exhibits a complementary strategy in the construction of NiPx‐based electrodes and offers bright opportunities to produce scalable hydrogen effectively and stably in alkaline corrosive electrolytes.