[OuNiS]

Yes it's possible. As I remember I read about that situations in Blair's "Hitlers's Uboat War" for example (or few Edwards's books). Some ships may sial under the balast (such wood or (better) cork, which is lighter than wood) and in this case they are harder to sink. Another way is a construction of the ship. Older tramp steamers are generally better to sink than modern freighters.

Ok. You must remember that I talked about real life situations not in a game life!