Weird as it sounds, Leif Ristroph of New York University decided that the aerodynamics of jellyfish offered a good method of designing a flying robot while keeping the size down. Air and water are both fluids, after all — the latter is just thicker.
Another issue, though, was stability. Flying insects — flies for instance — are inherently unstable. The fly has to do a lot of fine control. Nature has equipped the fly with some pretty sophisticated systems in its brain. But cramming all that software into a package small enough to fly isn’t easy. Hence the move to jellyfish as a model.