Why can't spaceships go underwater?
The trouble I see with that question is a lack of criteria for the desired answers: It doesn't quite ask for the design a spaceship which can go underwater; it doesn't ask how to prevent anybody from building a spaceship which can go underwater.
It simply states that many spaceships, to the knowledge of the asker, cannot go underwater, and then asks why.
See, it almost is a worldbuilding question: it seems to ask for the reasons why a spaceship cannot “go underwater”. However, it bases this on putative examples in real world engineering or in science fiction — not in its own worldbuilding.