Water is an attractive dielectric for the intermediate storage of electrical energy in compact pulsed power systems. A high relative permittivity of about 80 and a high dielectric strength of more than 1 MV/cm, allow for energy densities greater than 3 J/cm3. However, for very high field strengths the relative permittivity of water is no longer a constant. Early analytical calculations and more recent simulations predict a rapid drop for fields exceeding 1 MV/cm. Changes of the dielectric permittivity can also readily be seen in a change of the index of refraction which in return offers a possibility to evaluate the temporal and spatial distribution of the applied electric field. Measurements on the dielectric permittivity at low frequencies are difficult for high electric fields. We investigated the changes in the refractive index of water for light in the visible spectrum at 632.8 nm (4.74·1014Hz) in a pin-to-plane electrode geometry by means of a Mach Zehnder interferometer. Voltage pulses as short as 20 ns and up to 90 kV were applied. The applied voltage corresponds to a maximum electric field of 4 MV/cm in the gap. A high speed camera with a temporal resolution of 3 ns recorded the fringe patterns. To assess changes of the refractive index we had to distinguish between different effects as they arise from the Kerr-effect and changes in temperature. The results show that for electric fields of more than 3 MV/cm the theory of the Kerr-effect can no longer sufficiently describe the experimental results suggesting a strong polarization of the medium.