Ternary nanocomposites based on carbon fiber (CF)/polyoxymethylene (POM) composites and further reinforced with different amounts of alpha-phase alumina (Al2O3) nanoparticles were fabricated. The results show that the incorporation of Al2O3 nanoparticles simultaneously enhances mechanical strength, thermal stability and seawater resistance, and optimal properties were obtained at 4.0% Al2O3 nanoparticle loading. Remarkable enhancement of tribological properties in seawater at ordinary pressure was attained, while elevated seawater pressure weakened the wear resistance of Al2O3/CF/POM composites. At ordinary seawater pressure, the tribological properties of Al2O3/CF/POM composites were revealed to be governed by the transfer behavior between polymer-metal interface. At elevated seawater pressures, however, the transfer of Al2O3/CF/POM composites prevents the metallic counterpart from corrosion-wear damage at the expense of increased wear loss.