There are plenty of flying and swimming drones, but you'd ideally have both at once for sub-hunting -- you want something that can poke its head underwater, but move quickly through the air when needed. The US Navy certainly knows this. It's developing a duck-like drone, the Flimmer, that can both fly and swim. In addition to both a rear-facing propeller and wings, its latest incarnation has four fins that adapt to what the robotic craft is doing. In flight, they serve as stabilizers and canard wings; in the sea, they flap to give the machine a speed boost.