A huge dielectric effect has been observed in a pure and water-soluble hydrogen-bonded organic crystal, 1,4-diazabicyclo[2.2.2]octane hydroiodide [C6H13N2]+.I(-) (dabcoHI). In this structure, the dabco cations are NH+...N bonded into linear aggregates, where the protons are disordered at two nitrogen atoms and the crystal acquires the symmetry of space group P6m2. This nonpolar crystal exhibits a barely temperature-dependent dielectric constant exceeding 1000 at ambient conditions. The dielectric response is extremely anisotropic, more than 2 orders of magnitude higher along the linear hydrogen bonded chains than in perpendicular directions. The physics underlying this effect originates from proton transfers in the NH+...N bonds, leading to disproportionation defects and formation of polar nanodomains, which, on the macroscopic scale, results in one-dimensional relaxor ferroelectricity. Such properties are unprecedented for the materials with hydrogen bonds highly polarizable due to proton disorder. The proton disordering in dabcoHI is analogous to this in H2O ice, where the hydrogen bonds remain disordered until the lowest temperature.