Well, a team at Johns Hopkins University has created a UAAV – an unmanned aerial-aquatic vehicle. Named after the creature that inspired it, the Flying Fish is capable of both fixed-wing aerial flight and underwater travel.
Developed over the past two and a half years by robotics researchers Joe Moore, Eddie Tunstel and Robert Osiander, the Flying Fish is propelled in the air and under the water by a single motor and propeller. That prop spins quickly while the UAAV is airborne, but then slows down to a more appropriate speed when it dives below the water's surface. It speeds up again as the vehicle is about to return to the air, providing sufficient thrust to launch it up and out of the water.