Harvard's Robobees are already pretty amazing inventions for being so small, and they can fly around a room and hover like a real bee can. Unlike a real bee, however, Robobees can now "fly" underwater.
To be able to swim, or "fly" underwater, the Robobees crash-land into the water, after which they sink a little. They the flap their wings a little slower than they would in the air, flapping at 9 Hz underwater, compared with 120 Hz in the air.
While other drones can already swim, the goal with Robobees is to eventually build a robot that can do search and rescue, among other things.
In fact, the team was able to perform the new skill without modifying existing Robobees, however, there were, of course, a few caveats. For example, the bees still aren't able to fly without being connected to a power source. Researchers also had to lubricate the wings of the robot, otherwise, it would be too light to break the surface tension of the water and properly transition from flying to swimming. Last but not least is the fact that the drone needed a little extra energy to fly out of the water and back into the air, and after emerging from water, it didn't fly too well for a few moments.
These issues still need to be sorted out by scientists at Harvard, as do a few others that don't generally phase bees in real life, like how to fly in heavy winds.