With the development of marine renewable energies, the durability of materials at sea is more than ever a major issue in reducing the risk of failure of offshore devices. In this aggressive environment, elastomers, and in particular polychloroprene (CR), have many applications because of their good damping and fatigue properties. However, these materials are subject to ageing in service, which leads to changes in their mechanical properties. The ageing of these materials in a marine environment has not been extensively studied, despite the need to predict components lifetimes. This paper investigates the mechanical and microstructural consequences of a carbon black-filled CR degradation when exposed to seawater. To this end, swelling, uniaxial tensile and fatigue tests are carried out on materials previously subjected to accelerated ageing in natural seawater. Particular attention is paid to understanding the physico-chemical phenomena involved, and analysing fracture and fatigue properties in relation to those of the macromolecular network.