Most people use quadcopters for aerial video. Drones could prove useful underwater as well. Take the Loon Copter for instance. It is a multi-rotor vehicle capable of flying in the air and performing missions underwater. It can also float on the surface of the water with its sensors below the surface.
Since the drone is buoyant, it does not require any energy to float. It has 4 motors to maneuver underwater. The above video shows Loon Copter in action.