Well, since the 1.01b patch, the soviet subs take a lot of killing, that's for sure. In 1.0 one Mk-48 would usually end a Victor's active career. In 1.01b it takes at least two to shut them up.
They also keep firing even after blowing emergency ballast or sinking to the bottom. I once steered my Mk-48 into the nose section of a Victor-I in order to disable its tubes, but to no avail. It just kept on spitting fish at me.
On another occasion, I managed to put three Mk-48s in a Typhoon and it sat on the bottom but still launched two fish at me. I took one hit and managed to lure the other into the hissing wreck of the Typhoon. That did him in in the end
