Electrical breakdown of tap water (350 μS·cm-1) under sub-millisecond pulses (0.5 ms) with a gap distance of 3-35 mm has been investigated. It was found that the gap distance has a great influence on the polarity effect of underwater electrical breakdown, including breakdown mode, breakdown voltage, and pre-breakdown time delay. Under positive voltage impulses, the breakdown voltage increases with the gap distance; meanwhile, the pre-breakdown time delay decreases, and both of them show a saturation trend. Under negative voltage impulses, the breakdown voltage and pre-breakdown time delay increase with the gap distance, and the pre-breakdown time delay shows a saturation trend. Consequently, the polarity effect on the breakdown characteristics is varied with the gap distance. Further investigation by high speed optical observation confirmed that the polarity effect variation is closely related to the breakdown mode transition of positive polarity with the gap distance. Finally, a simple model based on the linear propagation assumption was proposed to predict the variation of the breakdown voltage with the gap distance.