In this research, we report the use of a commercial epoxy based nanocomposite with 2 wt % nanosilica to enhance the fracture toughness and wear performance of neat epoxy under marine environment. The mechanical behaviours of neat epoxy and the nanosilica enhanced epoxy composite are characterised after different periods of immersion in sea water (0 day, 7 days and 30 days). The experimental results indicate that sea water immersion does not greatly affect the tensile behaviours of the epoxy or the nanosilica enhanced epoxy nanocomposite. However, the fracture toughness of the neat epoxy is weakened by the sea water immersion with an 18.5% decrease after 7 days immersion and no further decrease after 30 days immersion. The fracture toughness of the nanocomposite also decreases after immersion, but the final value after immersion is comparable to that of the neat epoxy without immersion. The friction coefficient of the neat epoxy increases but that of the nanocomposite decreases with increasing immersion in sea water. The proposed mechanisms are also discussed here.