Astronomers Detect a Possible Signature of Life on a Distant Planet

It would probably be useful for high speed travel. Speedboats can go a lot faster than submarines. Airplanes can go even faster, but they might be a bit impractical since they'd have to be full of water.

Also radio communications since radio travels much better through air than through water. Of course, they could just build antennas underwater and then float them above the surface to use them.

Navigating by the stars would be really useful, and for that you need to be able to see above the surface. I guess you don't need to go above the surface. A periscope would do the trick.

It might also be useful for energy generation. Wind power if the wind is stronger than ocean currents. Or solar because the light is stronger.

Their airplane doesn't necessarily have to be full of water, just like our high altitude (military) planes and spaceships do not have to be full of air. For convenience, yes, but just for survival you'd wear a protective suit with life support systems, just like we do. Edit: But I agree that they'd be impractical for convenient business and leisure travel for the average octopus out there...

But with a full water atmosphere in a space ship you would be way more protected from radiation :)

> Also radio communications since radio travels much better through air than through water.

There is https://en.wikipedia.org/wiki/Whale_vocalization which carries rather far, if not disturbed by humans.