Flying, from a physical point of view, is quite similar to swimming. In both cases it is a matter of floating and steering in a desired direction by acting on the surrounding fluid (air vs water).
However, from a mechanical point of view the difference couldn't be greater. Air is "thin" and you need plenty of wing/sail real estate to float. On the other hand water is "dense" and you need to minimise surfaces to decrease drag.
Hence, creating a vessel that can be at ease both in flying and in "swimming" (on or in) water is quite a challenge.
In Nature we see that animals had to specialise to become proficient either in swimming or in flying. With a very few exception, like the puffins, very nice birds I had the opportunity to photograph in Iceland. The trick of the puffin is to have wings that can move in quite different ways and either become fins like or spread out the feathers.
Now researchers at Harvard have perfected one of their microrobot, the RoboBee, giving it the ability to fly AND swim, on the water surface or under it, behaving like a submarine (take a look at the clip).
The first problem they had to solve was the very light weight of the RoboBee, so light that it would float, staying on the surface of the water. To become a submersible it has to fold its wings before hitting the water surface and has to enter water at an angle to decrease the surface tension.
Once submersed another problem arises. The water density is a 1,000 times higher than the air density and that would break its wings. Hence once in the water its flapping frequency is reduced from 120 to 9 per second.
The RoboBee is so small that it cannot store the required energy "on board" so it is tethered by a cable providing the needed electrical power. Of course electricity and water do not go hand in hand and the researchers had to devise a special insulation trick.
So now they have a tiny robot that can fly and can jump into water to become a submarine. The reverse, emerging from the water and start flying, is not yet possibile. The drag on wet wings is too big for the limited power available, but researchers are working on it.
So far there is no clear application area but the challenges that need to be solved may provide solutions in other fields. Or may be, as the researchers are joking, the next 007 movies will see a flying and submersible Aston Martin...