Abstract: OUS (overwater unmanned system) mainly include USV (unmanned surface vehicle) and UAV (unmanned aerial vehicle), which are of great significance in the investigation of high-risk areas such as shallow water and islands, which are difficult to reach by traditional big vessels and surveyor. The purpose of this paper is to discuss the features, reliability, and effectiveness of OUS in island investigation. C400 acoustic USV and Phantom 4 RTK photographic UAV were used to acquire multi-beam sounding, side-scan sonar, and DOM (digital orthophoto maps) data in the Dongluo Island, Hainan. The results show that the collected data can clearly identify the submarine geomorphology of the island such as submarine dunes, sand ripples, sand pitches, and reefs, as well as the land geomorphology of the island such as bare rocks, vegetation, houses, docks, abandoned iron boats, and artificial plank roads. It is confirmed that UAV orthophoto plays a good auxiliary role in the survey of USV, and the OUS joint survey can realize the complete integration of land and submarine geomorphological data of the island. The collected data have high reliability and good usability, which are of great reference value for marine pasture construction, reef ecological restoration, and revealing features of island geological disasters.