A dielectric is a material that will polarize itself when subjected to an external electric field. The dielectric material itself is not a true conductor of electricity. But the partial positive and partial negative charges within the dielectric will orient themselves with the applied electric field. You may be thinking that water is a good conductor of electricity but remember that only water with ions present conducts electricity. Deionized water (water with all ions removed) is actually a poor conductor of electricity. The dielectric constant of pure water at 20 degress C is 80.1.