Dynamic mechanical and dielectric spectroscopies have been used to investigate the relaxation behavior of Polymethylmethacrylate (PMMA) both in the dry and in the wet state. Absorbed water shifts the main glass transition to lower temperature and contributes two loss processes (gamma and beta') in the glassy state. The location and activation energy of the low temperature water-related relaxation (gamma) are typical of local molecular motions. Conversely, the second water-induced absorption (beta') is characterized by high values of the activation parameters, suggesting that the dissipation process originates from cooperative relaxation phenomena.