Leakage of reservoirs will not only lead to the waste of water resources, but also cause soil salinization, dam break, and other serious consequences. Leakages may still occur even anti-seepage engineering measures are taken. This study utilized continuous waterborne electrical resistivity tomography to localize potential bottom geomembrane leakage areas in Datun plain reservoir in the east route of South-to-North Water Diversion project. The integrated sensitivity was firstly evaluated to select the optimum electrode array. Four leakage scenarios were numerically simulated and the resulting resistivity distribution patterns were further categorized to identify leakage locations. A total of 33 field survey lines were measured using floating cables over 5 km2 water surface. Robust inversion method was adopted for delineation of geomembrane interface. In the end, we found most of the soil surfaces are still kept under original flat conditions. Based on the abrupt decrease of resistivity from over 120 Ω·m to 80 Ω·m or lower values, two specific sites under the geomembrane with an area of 0.31 km2 potential leakage areas were localized. Even though there are no clearly leakage signs observed from the reservoir water surface, these resistivity results provide important preliminary data to further narrow down the leakage locations.