Rubber products are widely used for marine applications, such as fenders, bumpers, ship launching airbags, and hovercraft skirts. Many of these products are made through a vulcanising (crosslinking) process where large-scale tooling for high-temperature and high-pressure moulding is required. The cost will be greatly magnified when manufacturing large-scale components, such as flexible parts for next-generation wave energy converters. Two-part polyurethane rubber (PUR) is a room-temperature, low-pressure curing alternative that could minimise manufacturing costs. However, the general inferior mechanical performance of such room-temperature-vulcanising (RTV) rubber (compared to vulcanised rubber) restricts the application of PUR in the marine industry. It has recently been reported that the addition of graphene oxide (GO) could significantly improve the internal bonding of polymer elastomers and thus provide better mechanical performance with a low filler loading. In the work presented in this paper, different types of silane coupling agents (SCA) were used to treat the surface of GO, and the potential application of PUR/GO composites under marine environments and their mechanical performance were investigated. More than 75 % increases in tensile strength were observed after 1 wt % of GO was added. Moreover, a significant reduction in water absorption occurred during the seawater immersion test. It is suggested that hydrophobic sites provided by silane coupling agents play an important role on the GO surface, where polar groups, such as hydroxyl and carboxyl groups, were replaced during the silane treatment.