Moisture absorption in carbon fiber vinyl ester reinforced composite facings used for marine composite sandwich structures can be approximated by the Fickian diffusion model. Comparative study of water uptake for VARTM based polymer composites for exposure to tap, distilled, and sea water is presented. Evaluating moisture diffusion coefficient based on one-dimensional Fickian model usually involves errors resulting from large scatter of weight gain data at early times of exposure due to very small changes in moisture content of polymer composites. A novel measurement technique is proposed here for precisely measuring moisture absorption–desorption curves with improved precision to evaluate the diffusion coefficient. Mechanical properties of carbon fiber composite facings in terms of modulus and failure stress and associated degradation due to long-term sea water exposure are also summarized corresponding to different ply lay-up orientations. The matrix dominated lay-up orientations show considerable degradation in mechanical properties due to sea environment. Notably, tension–tension fatigue on matrix dominated composites after sea water saturation resulted in 30 % degradation of fatigue life.