A study of the electrical breakdown of water under microsecond pulsed voltages has been performed. The influence of conductivities (350-1550 μS/cm) has been reported in the needle-plate configuration. Both positive and negative applied voltages (10-30 kV) have been adopted to study the streamer propagation and breakdown phenomena. Three streamer modes are observed in our experiment, and the morphologies of streamers at different conductivities are compared. Increasing the conductivity leads to a thinner gaseous layer and stronger luminance for all streamers. However, the polarity effect of the applied voltages causes differences in the mobility of charges in water and results in different propagation velocities. The minimum breakdown voltage at different polarities exhibits different trends for positive subsonic streamers and negative subsonic streamers. This paper has evaluated the shock wave intensity and bubble dynamics after the breakdown of the water gap. The results show that the relationship of the peak pressure (Pm) and the residual energy of the capacitor at the breakdown time (Wb) is consistent and stable. The bubble dynamics and shock wave propagation are less affected by the water conductivity.