Photocatalytic hydrogen (H₂) production from seawater over a photocatalyst is of great significance for the utilization of theearth’s abundant seawater resources and the storage of inexhaustible solar energy with low-energy density. However, the designation of efficient photocatalytic systems by using seawater usually suffers from activation decline when replacing pure water with seawater. Herein, we synthesized three β-ketoenamine-linked covalent organic frameworks (COFs), demonstrating their promotion in the photocatalytic decomposition of seawater relative to pure water under visible-light irradiation. The target COFs were synthesized via a microwave assisted solvothermal method by using 1,3,5-triformylphloroglucinol (Tp) as the aldehyde monomer to react with diamino units with different numbers of benzene rings. By surveying the building blocks, the constructed COF with a high density of β-ketoenamine units exhibited the so far highest photocatalytic H₂ evolution rate (41.3 mmol·g⁻¹·h⁻¹) in seawater, about 1.66 times higher than that in pure water. The β-ketoenamine units allowed the in-situ polarization of the COF framework through the adsorption of metal salts when proceeding with the photocatalytic H₂ production in seawater. This polarization effect significantly increases the dielectric constant of the organic semiconductor to lower exciton dissociation energy and thus enhances the charge separation and transfer to promote the H₂ photoproduction in seawater.