Composite materials are used in many marine structures and new applications are being developed such as tidal turbine blades. The reliability of these components, in a very severe environment, is crucial to the profitability of tidal current energy systems. These structures are subject to many forces such as ocean tides, waves, storms but also to various marine aggressions, such as sea water and corrosion. A thorough understanding of the fatigue behavior of the moving parts is therefore essential. The majority of tidal turbine developers have preferred carbon blades, so there is a need to understand how long immersion in the ocean affects these composites. In this study the long term behaviour of different carbon/epoxy composites has been studied using accelerated ageing tests. A significant reduction of composite strengths has been observed after saturation of the material in seawater. For longer immersions only small further changes in these properties occur. No significant changes have been observed for moduli nor for composite toughness. Changes in properties are initially due to matrix plasticisation, which increases failure strain, followed by reductions due to fibre/matrix interface changes. The evolution of the rate of water ingress into composite materials is important to follow, in order to develop predictive models of property changes through the laminate as a function of diffusion kinetics.