Numerous investigations on plasma–liquid interaction have been published in the literature using various reactor geometries. However, our understanding of the basic configuration of a pin-to-plane dielectric barrier discharge (DBD) in contact with water is still incomplete. We, therefore, performed the electrical diagnostics of such a discharge, operated in an AC regime with sinusoidal high voltage, focusing mainly on the power injected into the plasma in relation with several experimental parameters. It was found that the injected power increased linearly with the amplitude of the applied voltage, while without water, the evolution followed a second-order polynomial. In both cases, the discharge power was proportional approximately with the frequency. For the DBD above water, the power was about three times greater than that without water for the same air gap. It was observed that changing the discharge gap from 1 to 5 mm led to a slight increase in power by only 15%. The water conductivity also influenced the power injected into the plasma only to a small extent. Thus, the variation in water conductivity over four orders of magnitude determined a fluctuation in discharge power of ±20%.