'just below the surface' -> already at very impressive pressures and given the complex shape of their hulls absolute marvels of engineering. 240 meters below isn't 'just below the surface', that's pretty deep. That the ocean is much deeper is a simple fact but that should not diminish your appreciation for the differences between a 'regular' submarine and a deep sea vessel. Which is also why the latter tend to be extremely simple geometric shapes for the pressure vessel because of the stresses they are subjected to. Ball shape preferred, then cylinder. Something with angles in it would fail for sure.