The dynamics of hydration water at the silica interface of elastomer (S-SBR) composites filled with various types of silica with and without activation by silane is investigated by broadband dielectric spectroscopy. this gives some new insight into the mechanisms of silane coupling and polymer-filler interaction in silica-filled high-performance elastomers. Depending on the hydration level of the silica, water islands or complete coverage of the silica surface with at least one monolayer of water are identified by the dielectric response. The observed two low-temperature relaxation transitions are shown to be related to fluctuations of physically adsorbed water (SP1) and of hydroxyl groups (SP2), respectively, differing in relaxation strength and activation energy. The relaxation strength of the main peak SP1 decreases with silanization, indicating that, due to hydrophobization of the silica surface, part of the adsorbed water migrates into the rubber matrix. At low surface coverage, below one monolayer of water, the activation energy of both relaxation transitions is found to increase with the hydration level due to an increasing mean number of interacting hydrogen bonds in the merging water islands (SP1) and a rising number of water-hydroxyl interactions (SP2). Above one monolayer of water, the activation energy becomes almost independent of hydration level. For nonactivated silica composites (without silane), the activation energy of the main peak SP1 is close to the value 0.6 eV, also observed in ice. Modification of the silica interface by silane increases the activation energy tip to about 0.7 eV, indicating that hydrophobization and chemical Coupling of the polymer chains to the silica Surface reduces the mobility of the hydration water. This opens the possibility to assess and predict the coupling strength of the polymer chains to the silica surface by measuring the activation energy of the hydration water of the composites.