This farfetched idea came while reading the section "Bathtub aerodynamics" in an old (but excellent) science experiment book :
https://archive.org/details/TheAmateurScientist

The fact that model airplanes can fly well in water made me think of doing the same with full-scale planes. And then I started imagining an underwater version of the Battle of Britain, with submarines playing the role of bombers intercepted by underwater-adapted airplanes flying from bases on the coastal seafloor...

Of course, the hydrodynamics effects are completely different at full-scale and the modifications needed would be too extensive to be worth it. But this concept is just too cool to pass on.