[potato3732842]

A submarine needs to be light to be neutrally buoyant in order to fly though the water properly. Otherwise you have a bathyscaphe, which has some other not nice failure modes (in addition to the "implodes if you f it up" one) and are much less maneuverable and arguably the whole system is less durable more costly for high tempo operations. "Just build a tube strong enough and big enough to not need all that" is a better answer, if you can pull it off.