Photocatalytic hydrogen production on semiconductors has been intensively researched in the past years, yet it still lacks research on efficient and stable photocatalysis in seawater. In this paper, with cadmium sulfide (CdS) as a sample photocatalyst and carbon dots (CDs) as cocatalyst, we fabricated a series of CDs/CdS composites serving as efficient hydrogen production photocatalysts in both water and seawater. In this case, the optimal CDs/CdS-S composites (nanosheet with cocatalyst content of 0.01 g(CDs)/g(catalyst) exhibit prominent H-2 production rate of 4.64 mmol h(-1) g(-1) (6.70 mmol h(-1) g(-1), water) and AQE of 11.8% (19.3%, water) under 420 nm light irradiation in seawater, which are about 265 and 169 (78 and 77, water) times of those of the non-modified irregular CdS in seawater, respectively. The excellent activity and stability are attributed to the competency of CDs that not only dramatically improves the charge separation efficiency, but also plays indispensable role in resisting the distraction from various ionic components in seawater. We hope our work can provide a feasible perspective for constructing high-efficient photocatalyst towards practical applications.