I think you have a misconception on how wartime ships went under. Some sank catastrophically, like the HMS Hood, and the few survivors where those both protected by the explosion and high up enough to be able to get out of the sinking ship.

However, ships exploding like that was uncommon - much more common was a slow sinking fought by the damage control teams of the ship.

In such cases, the men in the deep of the ship were the most protected and would have a good chance to survive, while those in the unarmoured superstructure of the ship would be most subject to enemy fire.

Many of the 116 who survived Bismarck's sinking were machine crew who had been in the deepest parts of the ship.

When the HMS Glorious was sunk by the German battleships Scharnhorst and Gneisenau, there were about 900 survivors who entered the water, but only about 40 were ever picked up - so the great loss of life happened after the loss of the ship.