The development of high-performance and robust non-precious metal-based catalysts to accelerate electrocatalytic reaction kinetics is crucial for electrochemical seawater splitting. The common electrocatalysts for seawater electrolysis suffer from sluggish reaction kinetics and instability, restricting their practical application. Herein, nickel–iron layer double hydroxide and Ni3S2 heterostructured nanoflower bifunctional electrocatalysts (NiFe LDH-Ni3S2) were synthesized via seawater corrosion and ion exchange strategy. The desert rose shaped nanoflower structure of the catalyst was tuned by seawater corrosion time. The unique heterostructure not only possessed more redox reaction centers, but also enhanced anti-corrosion ability, which could effectively facilitate mass diffusion, charge transport and maintain high electrocatalytic activity in seawater. The as-prepared NiFe LDH-Ni3S2 electrodes recorded overpotentials as low as 257 and 280 mV to deliver 100 mA cm−2 for HER and OER in 1 M KOH seawater, respectively. In 1 M KOH seawater + 0.33 M urea, there was an obvious reduced urea oxidation reaction (UOR) potential of 1.37 V to reach 100 mA cm−2, which is about 140 mV lower than for OER. Noteworthy, Ni/Fe LDH-Ni3S2 as a bifunctional electrocatalyst featured a low voltage of 1.63 V to reach a current density of 100 mA cm−2 and almost 100% faradaic efficiency for overall water–urea splitting, which is superior to the most previously reported electrocatalysts. This work provides insights on the application of seawater corrosion engineering technique to modulate the electrocatalytic activity in energy conversion and seawater utilization systems, achieving efficient conversion of solar energy, electricity and hydrogen.