Due to the special nature of the aquatic environment, the placement of electrodes for electrical exploration is difficult and the data collection efficiency is low. Currently, 2D electrical exploration is mainly used in aquatic areas, making it difficult to accurately locate the target bodies. 3D DC exploration is rarely reported. Based on the fast acquisition technology of parallel electrical exploration and the joint 3D inversion method of multiple survey lines, this article conducted a 3D DC electrical exploration of the original bottom boundary of a quarry in aquatic conditions. A numerical model similar to the actual geological conditions of the survey area was constructed, and the feasibility of 3D DC electrical exploration was verified through numerical simulation. During actual exploration, electrodes were arranged on the water surface, and a pseudo 3D observation system data composed of multiple 2D survey lines was used for 3D inversion. By obtaining multiple XZ resistivity profiles, the depth data of the quarry bottom in a 30 m×5 m grid was obtained, forming contour maps of the quarry bottom boundary. The accuracy of the results was verified through drilling calibration, satellite photo comparison, actual pumping exploration, and other means.