We report an ab initio Molecular Dynamics study of the hydration process in a model IRMOF material. At low water content (one molecule per unit cell), water physisorption is observed on the zinc cation but the free ⇄ bound equilibrium strongly favors the free state. This is consistent with the hydrophobic nature of the host matrix and its type V isotherm observed in a classical Monte Carlo simulation. At higher loading, a water cluster can be formed at the Zn4O site and this is shown to stabilize the water bound state. This structure very rapidly transforms into a linker-displaced state, where water has fully displaced one arm of a linker and which corresponds to the loss of the material's fully-ordered structure. Thus an overall hydrophobic MOF material can also become water unstable, a feature that was not fully understood until now.