The influence of hydrostatic pressure on composite behaviour is critical for many underwater applications. Under these extreme environmental conditions water diffusion 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, glass reinforced epoxy composites). First, water diffusion was studied at different pressure levels (1, 50 and 500 bar) in order to identify the diffusion models. For the neat epoxy resin the water uptake remained unchanged by pressure rise, whereas glass fibre reinforced epoxy had a maximum moisture content that increased while the diffusion coefficient was unaffected. In a second part, the present study focuses on the identification of the diffusion law using a numerical method. In the final section the analysis of microstructure revealed one influent parameter on the water diffusion in composites under pressure. Indeed, an X-ray micro-tomography study showed a high level of porosity in the glass fibre reinforced epoxy. Moreover, glass fibres are hydrophobic which results in water diffusion exclusively located in the resin and in voids in the composite. Whereas resin water uptake does not depend on hydrostatic pressure, the additional moisture content in voids showed a high dependence with pressure level and a link with porosity ratio in the composite.