I suspect if water friction/drag wasn't a thing (e.g. like being in space), sonar signature didn't matter, there was no need to consider ricochet or deflection physics, thermal factors... you know, all the stuff they have to consider in the real world but probably aren't in this game... then external shape would be somewhat irrelevant.

Just make it whatever shape makes you feel awesome when piloting the thing!

In the real world I think current thinking is that whale or torpedo shaped is good.

In terms of practicality, I much prefer to have all of the important stuff on the same deck, the only things I want to have to use a ladder for are non-essentials such as fabrication, ballast tanks, and airlocks. If you prefer to have something more like a space station with many different levels and rooms, then shape is kind of irrelevant. Although, it is kind of cool and fun to have quirky little rooms almost hidden away in one corner of the boat.

Generally speaking, a round-ish shape is much, much easier to cover with gun arcs, offers lesser chances of being hit by its own fire if said arcs are a bt wider than normal and offers less nooks and cranies for monster to hide.

Do note that having nooks and crannies isn't toally bad, as a good captain can "hold" a few corpses with a stationary sub for easy looting, and this isn't easy to do when you're piloting a soccerball.

VERY generally speaking.

As a final note, the best submarines are the ones that maximize !fun in MP and reduce hassle and management in SP, imho. Shape is secondary at best.

I'll echo Buggy Boy's statement in saying that flat subs are better than tall subs. This becomes more important the larger your sub is, as human characters move much faster horizontally than they do vertically.

Room placement is also very important. If the command room is close to the outer hull, there's a stronger likelihood that the captain will be pushed around or drowned if there's ever a breach, which can make a bad situation worse. The same could be said of the gunnery compartment.

Low-slung rooms are also much more prone to prolonged flooding than rooms higher up on the sub. I'd say this is more of a trade-off. The longer a room is flooded, the more likely it is to become pressurized, but it also means less rushing water to knock people around, and people are more likely to find diving suits sooner if they have to stay in that room.

No jagged edges.
Corners and sides are rounded and smooth.

Jagged corners and sharp edges are structural weak points in submarine hulls. Your in game ship will look more realistic if you prevent yourself from allowing any of your shell parts to become pointed.

I've made one submarine and there is not one point where the hull becomes jagged. Every time I needed to cap something or work around something I always asked if that would look realistic or not.