Using terahertz time-domain spectroscopy (THz-TDS) the complex dielectric response of mixtures of the prototypical ionic liquid [1-butyl-3-methylimidazolium]+:[BF4]-([bmin]BF4) with water is investigated. The dielectric response can be described well using a two-term Debye model, and these two relaxation processes in the THz region are followed as a function of volume fraction of the constituent liquids. For the mixture, slow and fast Debye relaxation times are larger than the values expected from interpolation of pure water and pure ionic liquid values, pointing to increased structural coherence within the mixture.