that is the thing here. While the project is cool, the drone's underwater drag is very high, while typical such drone of that size is foldable, and thus one can fold it (even if a bit, Star Wars X-wing style) to decrease drag and use say only one or two motors with much greater efficiency as result. Though high speed drone like this https://www.youtube.com/watch?v=7PTsHDFMams may be ok without collapsing.

But you use spend much less energy fighting gravity.

I’d expect it to have more range underwater than a typical quadcoter has through air. And much longer “flight” time.

But I doubt it gains enough to compete with a fixed wing drone using the same battery.

>I’d expect it to have more range underwater than a typical quadcoter has through air.

I would expect the opposite, with the higher drag being much more of an issue than gravity. But I would be interested to hear a definitive answer.