Water diffusion under hydrostatic pressure is critical for many underwater applications. Nevertheless it has rarely been studied, and published data are contradictory. The aim of this study is to understand what governs pressure effects by studying different materials (unreinforced resin, and three glass-fibre reinforced epoxy composites). First, kinetics of water diffusion, for unreinforced resin and composite materials, are identified at different pressure levels (1, 50 and 500 bar). For the neat epoxy resin the water uptake remained unchanged when pressure was raised. The glass fibre reinforced epoxy composites produced by hand lay-up have a saturation level that increases significantly with increasing pressure, while the diffusion coefficient is unaffected. The infused composites show only a small effect of pressure slowing initial diffusion rate, while the prepreg composite show no effect. In a second part, the present study focuses on the identification of the diffusion law using a numerical method. In the final section X-ray micro-tomography is used and reveals a high level of porosity in the hand lay-up composite. Moreover, as glass fibres are hydrophobic and resin water uptake does not depend on hydrostatic pressure it is concluded that additional water diffuses into voids under pressure.