Deliberate qua skewspeak.
Was triggered by this article, with the pic of a sub. That is waaay over size, when one would consider not needing the space for hoomans (and no need for much oxygen ..?); redesigning a drone sub from scratch might give quite different results.
Smaller, probably. More flex. Less visibility, in all sound / electromag fields. Possibly, cheaper builds (no need to care for human safety ..?). Have a look at what free design did for aerial, fixed-wing drones à la Raptor.
Surely, the Navy already is experimenting with drones beyond the cable-attached, close-by controlled, kind ..? What about their designs, re-optimised (e.g., using genetic algos as here and here) with a considerably altered set of constraints or what?
Where are the pics ..? [Will sign an NDA…]
[Already off-average, even when classically designed… analog pic at Twente AFB, when that still was]