  We study water between parallel metal walls under applied electric field
accounting for the image effect at $T=298$ K. The electric field due to the
surface charges serves to attract and orient nearby water molecules, while it
tends to a constant determined by the mean surface charge density away from the
walls. We find Stern boundary layers with thickness about $5$ $\rm \AA$ and a
homogeneously polarized bulk region. The molecules in the layers more
sensitively respond to the applied field than in the bulk. As a result, the
potential drop in the layers is larger than that in the bulk unless the cell
length exceeds 10 nm. We also examine the hydrogen bonds, which tend to make
small angles with respect to the walls in the layers even without applied
field. The average local field considerably deviates from the classical Lorentz
field and the local field fluctuations are very large in the bulk. If we
suppose a nanometer-size sphere around each molecule, the local field
contribution from its exterior is nearly equal to that from the continuum
electrostatics and that from its interior yields the deviation from the
classical Lorentz field. As a nonequilibrium problem, we investigate the
dynamics after a reversal of applied field, where the relaxation is mostly
caused by large-angle rotational jumps after 1 ps due to the presence of the
hydrogen bond network. The molecules undergoing these jumps themselves form
hydrogen-bonded clusters heterogeneously distributed in space.
