It was not realistic to armour the underwater portion of a ship in the same way you did against shells, as the dynamics of a shell hit are quite different from those of a contact or non contact underwater explosion. Not only would the armour itself have been prohibitively heavy, but so too would the backing structure to carry it (if too weak, it would allow the plates to be shifted inwards causing flooding anyways). Since armouring was impractical, designers sought to limit the amount of flooding to outer areas of the hull and provided an inner armour material bulkhead or bulkheads to contain the flooding and prevent it from penetrating to the vitals of the ship. Outboard of this bulkhead was a large void area in the most basic design; in more elaborate designs this outer area was divided into alternate layers of void and liquid filled compartments. The void was intended to allow the explosion bubble to expand in a controlled manner, losing some of its force before impacting either a liquid loaded layer or the main torpedo bulkhead; the liquid loaded layer was expected to absorb more of the energy and to spread the stress forces over the torpedo bulkhead which would deform elastically without rupturing. Generally the hull plating was kept as thin as structurally possible to avoid or minimize a torpedo detonation's generating secondary splinters which might pierce the torpedo bulkhead.
Some designs worked well, some less so, and some were never put to the test of combat. The main problem was that the systems even when performing well amidships, might perform less well fore and aft where the width of the system narrows to follow the hull contours until it was no longer possible. Since it was recognized that a torpedo hit would cause off center flooding, quick counterflooding and the ability to pump large amounts of water or fuel to the unaffected side would help minimize loss of stability. However, here again, some ships such as the Iowas had excellent liquid transfer capacity, while others, such as Yamato, were less satisfactory. No matter how good a system was, it was recognized that enough hits in rapid succession along one side of a ship would probably lead to loss of stability and capsizing well before loss of buoyancy. It was also recognized that some areas were impossible to adequately protect, such as the bow, and most particularly the stern where propeller shafting of necessity penetrates the side defence system and causes an inherent achilles heel.
Their shoulders held the sky suspended;
They stood and Earth's foundations stay;
What God abandoned these defended;
And saved the sum of things for pay.