Dielectric relaxation spectroscopy is applied for the study of alpha-and Maxwell-Wagner (MW) relaxations of polyurethane (PU) coating on steel and gold substrates during a water exposition. The evolution of the alpha-relaxation time of the PU coating in contact with water can be characterized by four stages: (1) Relaxation time increases with starting of the water penetration. The presence of water molecules in the coating leads to an increase in charge carriers at the metal/polymer double layer which influences the cooperative molecular motions in the polymer coating. (2) Relaxation time decreases because of the plasticizing effect of the water in the polymer. (3) Decreasing of the relaxation time and ionic conductivity associated with formation and growing of blisters. Appearance of blisters gives way to an interfacial dielectric relaxation (MW) in the low frequency region. (4) Overlapping of the alpha-relaxation peak by MW effect when the coating is completely displaced with water. The role of the metal/polymer interaction on these stages is discussed.