When conducting underwater observations in coastal fisheries and other offshore commercial operations, efficiency and safety are important considerations. We developed a combined aerial and underwater drone system that carries and flies with an underwater drone (Remotely Operated Vehicle: ROV). It can fly quickly and automatically to the target area to carry out observations without using a surface vessel. This drone can be operated remotely through a 4G mobile network, with underwater navigation and observation. Specifically, after the aerial drone carrying the ROV arrives at the target area automatically, and having landed on the water, it separates from the ROV, and the ROV dives under the sea. After the underwater observation work has been completed, the aerial drone docks with the ROV and lifts off from the surface of the water and returns to the land base station. Through sea trials, it was demonstrated that this combined aerial and underwater drone system could perform underwater observation successfully.