I figured i would talk abit about the science behind how a ship sinks. Perhaps it will help those who are modding the game's damage system
As for ww2 torpedoes being weak, well thats not exactly true. They were more then capable of breaking small and medium ships in half via a magneticly triggered blast. And with impacts could put holes many feet wide into a ship. They did have alot of explosive power for their time (typicaly 280-300 kilograms or 660 lbs of Hexanite explosive and up). Often times though the reason why it took multiple torpedoes to sink a ship, was because either the torpedo malfunctioned and didnt fully detonate, or detonated early, or hit a section of deadspace in the ship. Or the compartment it did hit was not large enough for flooding that section to cause the ship to sink. Idealy you want to flood a large mostly empty compartment to force a ship to sink. Also keep in mind the reports from uboat captains are flawed, they often didnt see all the torpedoes detonate, and their field of vision is very limited. Looking at the wrecks of actual ships sunk by uboats is far more telling.
To sink a ship via flooding in the real world, you have to decrease its displacement so its lower then the ship's true weight. This meens flooding large sections of the ship. Of course there are many ways this can happen, from say flooding the bow of the ship so it lifts the stern up, which places strain on the keel and will eventualy snap it (and likely cause the entire ship to sink). Also secondary flooding can ocurr from part of the ship being submerged (ships are not air and water tight, and the deck and hatches arnt designed to take any sea preasure and often rupture when submerged). Fires can also easily sink ships by causing fatigue in the metal till the stresses of normal sailing break the ship up. Obviously magazines/cargo exploding can easily send a ship to the bottom, as can damaging or breaking the ship's keel (which is how a magnetic torpedo sinks ships, not by putting any holes into it directly but by overstressing the keel and hull from the upsurge of water). There are also many other minor factors.
Another big factor is of course the ability for air to escape from the flooding compartments to allow more water in. This can be easily demonstrated using a bucket with a lid, put a hole down low on the side with the lid on and the bucket will flood to the point of the hole and slightly more then that up to where the water and air pressure equalize. Take the lid off and the bucket will sink like a stone. Put a pinhole in the bucket and it will slowly sink down till its displacement < weight then it will go down. Because of this principle, damage to a ship above the waterline can have a real effect on it sinking or not when combined with damage below the waterline. The ability for air to escape or not is one of the 2 major factors (the other being the volume of water flowing in) in determining how long it takes a ship to sink.