A bit off-top but it might be of interest...
The boat with most casualties in one attack was ... Soviet S-13
under command of Alexander Ivanovich Marinesko
Under the command of Marinesko, then 32, on 30 January 1945, at Stolpe Bank off the Polish coast, S-13 sank the 25,484-ton German liner Wilhelm Gustloff
, overfilled with refugees, with three torpedoes. Recent estimates calculate that over 9,000 people were killed, the worst loss of life in maritime history
Soon after that, S-13 fired at the T-36, a torpedo boat that had come to the aid of the Wilhelm Gustloff. Despite being overloaded with 564 survivors from the Gustloff, the captain of T-36 was able to dodge the torpedo.
During the very same patrol, on 10 February 1945, S-13 sank another German transport ship General von Steuben. 3,300 refugees and injured soldiers from the ship died
, and only 300 survived.
Marinesko has maneuvred submerged for four hours, following the enemy by sonar. He was convinced that the target was the light cruiser Emden. This way, Marinesko became the most successful Soviet submarine commander in terms of Brutto Register Tonnage (BRT) sunk with 42,000 BRT to his name.
However, Marinesko was not awarded for this the Hero of the Soviet Union title: his commanders refused to trust reports regarding the scale of the hit; in addition, he was deemed a controversial person, "not suitable to be a hero".
During his next mission from April 20 to May 13 Marinesko did not conduct a single attack, although he was sent to an area with intensive traffic of enemy ships. This mission was evaluated as unsatisfactory.
After the hits were confirmed, he was awarded the (lowly) Order of the Red Banner. Marinesko felt personally insulted, and when staff officers came to present him the order on his submarine, he gave the order to submerge her.
Due to problems with discipline and his alcoholism, in September 1945 Marinesko was removed from submarine command and transferred to shore duty, with a lowered rank, and in November he was discharged from the Navy. In the next years Marinesko ruined himself totally. In 1949 he even landed for two years for theft in a Kolyma prison camp. He died in 1963 in Leningrad of an ulcer.
Marinesko was awarded the title Hero of the Soviet Union posthumously, in May 1990, on the occasion of the 45th anniversary of the victory in Europe. The Submarine Museum in St. Petersburg was named after him, and monuments dedicated to him were erected in Kaliningrad, Kronstad and Odessa.
He was somwhow a very controversial person - read about him here: http://en.wikipedia.org/wiki/Alexander_Marinesko
Es steht ein kleines, kleines Edelweiß
auf einer steilen, steilen Felsenhöh!
1 KP, 100 GJR