[ProtectOurFreedom]

A squarish section? Subs have round sections for a reason.

[PIF]

Ask Boeing or as we used to say Gerald McBoeing-Boeing

[Robert A Cook PE]

You can wrap a round pressure hull with a rectangular outer hull. Use the “corners” to hold external pressure vessels, torpedo tubes, air tanks, fuel tanks, sensors or periscopes and other stuff. See the (failed design) US Navy ASDS for example.

[Neidermeyer]

Round is stronger ... but the "box fish" actually has the least drag... very important for minimalist propulsion systems... the pressure hull underneath can be round.