The use of seawater electrolysis for hydrogen production faces several serious challenges including rapid deactivation of electrocatalysts through chloride anion (Cl-) induced corrosion. We have demonstrated that Ru nanoparticles possessing an abundance of surface hydroxyl groups along with borate species (Ru-BOx-OH) have high activity and stability as electrocatalyst for seawater splitting. The optimal electrocatalyst (Ru-BOx-OH-300) uncovered in the study displays an extremely high catalytic performance for both the hydrogen (HER) and oxygen (OER) evolution reactions in alkaline seawater (HER, 22 mV and OER, 235 mV @10 mA cm-2), as well as a low cell voltage (1.47 V) and ultra long-term stability (1000 hours@10, 50 and 100 mA cm-2) for overall seawater splitting. Furthermore, the Ru-BOx-OH-300-based anion-exchange membrane seawater electrolyzer requires only 1.73 or 1.95 V to reach a current density of 500 or 1000 mA cm-2, respectively, and exhibits excellent stability for 400 hours without obvious decay. The results of experiments and theoretical calculations reveal that the high water affinity of Ru-BOx-OH-300 caused by the presence of hydroxyl and borate species on the metallic Ru surface are responsible for the superb electrocatalytic performance, and that the borate species are the source of Cl- corrosion resistance. These findings open new perspectives for the design of high performing electrocatalysts for seawater splitting.