Computer simulations were employed to build an atomistic model of a highly cross-linked fully aromatic polyamide membrane relevant to reverse osmosis materials. The methodology used to construct the membrane accounts explicitly for the solid/liquid interface and enables the control of the membrane cross-linking degree. A membrane cross-linked to more than 80% was generated and further hydrated. Both density and cavity size of the hydrated membrane were found in good agreement with experimental values available for commercial membranes made from identical monomers (m-phenylenediamine and trimesoyl chloride). It was shown that the number of hydrogen bonds per water molecule decreases significantly inside the membrane with respect to the bulk phase and that confined water forms hydrogen bonds with some preferential interaction sites on the polyamide matrix. The analysis of mean square displacements of water molecules inside the membrane revealed a decrease in the water self-diffusion coefficient by an order of magnitude with respect to its bulk value. Similarly, a substantial increase in the relaxation time of water dipoles was observed under confinement. Finally, the dielectric constant of water was determined from the fluctuations of water dipole moments and was found to decrease significantly inside the membrane with respect to bulk value.