A quadcopter that can fly like an aerial drone, but also splash down into water and operate like a submersible, could be used for search and rescue missions or engineering inspections. This drone can fly in the air or swim underwater using a dual-speed gearbox for each of its four motors.
The prototype device called Mirs-X, which weighs 1.63 kilograms and is 38 centimetres wide. It can hover for 6 minutes in the air or dive underwater for around 40 minutes. Mirs-X is waterproof at depths of up to 3 metres and moves at up to 2 metres per second in water.
More information:
https://www.newscientist.com/article/2357125-diving-drone-can-switch-between-flying-and-swimming/