Well, the fate of many boats ist not yet clear. 64 years after the end of the war, history of some boats must be rewritten.
For instance U869
http://de.wikipedia.org/wiki/U_869
After the sinking of the ships in the canal by U480, tons of depth charges were thrown but these droppings were not targeted in any way.
This tactic was well known and should lead the commander to panic induced reactions, so to reveal his true position.
They knew that there had to be a boat but they did not know where it was.
In the documentation, which is worth seeing, the wrecked boat can be seen.
The damaga pattern looks more like direct detonation of a big mine close to the hull. To hit a boat with depth charges in that way must be a lucky punch. Whereas hedgehogs or similar weapons would never had destroyed the structure of a boat so severely.