The coupling between tensile stress and water diffusion in composites represents a major issue in many marine applications. Even if these two behaviours are well documented as separate subjects, there are still very few data on fully coupled mechanisms. The aim of this study is to understand what governs the coupled behaviour, by investigating the water diffusion in carbon/epoxy. The coupling is first evaluated on neat resin samples to characterise the matrix behaviour. Then the study focuses on composites with two types of woven fibre orientation: one at ±45°, in order to understand the coupling effects on materials loaded away from the reinforcement direction and another quasi-isotropic to obtain properties along the fibres. For each material the same approach was applied: first, the tensile behaviour, damage development and water diffusion were studied uncoupled. Then, semi-coupled effects were investigated. The final part of the paper combines both behaviours as fully coupled phenomena