A UAV developed by a Johns Hopkins University Applied Physics Lab team can stay underwater for months, waiting out of sight until it's called to duty. When the machine does get deployed, it can swim up and fly into the air, just like any other quadcopter. The team calls their creation the
KRAKENS CRACUNS or Corrosion Resistant Aerial Covert Unmanned Nautical System, made using additive manufacturing (3D printing) and other techniques. It can stay submerged hundreds of feet beneath the water's surface, thanks to its composite airframe that can withstand the pressure.
In addition, the engineers sealed its most sensitive components inside a dry pressure compartment. They also painted its exposed parts with commercially available coatings that can protect them against the corrosive properties of saltwater. Both measures seem to have worked well during their experiments: the drone the researchers kept in sea water for two months showed no signs of damage.
CRACUNS doesn't have any metal parts that can rust and malfunction in the water -- best of all, it's lightweight and doesn't cost much. Those factors make it a good candidate for big research or military operations. Not to mention, if the company/government agency/university operating a fleet of these machines lose some to inclement weather or high-risk missions, it wouldn't hurt too much.