This study deals with the dielectric spectra of mixtures of the ionic liquid 1-butyl-3-methyl-imidazolium (BMIM(+)) tetrafluoroborate with water at three selected mole fractions 0.767<or=x(H(2)O)<or=0.967. The focus lies on the comparison of experimental and computational data. On the one hand, a computational analysis permits a complete decomposition of spectra, both with respect to dynamical behavior (translation and rotation) as well as to composition of the mixture (cation, anion, and water). Thereby, not only the peak assignment in experimental spectra is enabled but one can also learn more about solvation properties. Of particular importance is the interplay of the dielectric constant and the conductivity representing a measure of collective rotational and translational motion. On the other hand, the comparison with experimental spectra is essential for the validation of the force fields used in simulation. The satisfying agreement between corresponding peaks in the dielectric spectra confirms not only computed dielectric relaxation times but also other collective dynamical properties such as the viscosity. Nevertheless, the detailed fine structure of the conductivity regime reveals specific ion-pair effects not covered by the simulation. A possible confinement of dynamical heterogeneity as a consequence of a system size effect is also indicated.