Drone technology is expected to play an important role in various fields, including topographic survey, wave and current monitoring, lifesaving, etc. in the ocean. In the present paper, we propose a new method for bathymetry, with the combination of an unmanned aerial vehicle (UAV) and an unmanned surface vehicle (USV). First, the motion of the USV is evaluated using images obtained by the UAV. Second, the pixel shift values of the seabed are calculated through underwater images by the USV. Finally, based on these results, the water depth is acquired by applying triangulation. The experimental results obtained using a wave basin have shown the maximum estimation error of 1.1cm, indicating the effectiveness of the proposed method.