^ Yet at the Nuremberg trials the prosecution of Dˆnitz for the Laconia order was mostly abandoned, partly due to the testimony of US admiral Chester W. Nimitz that US submarines in the Pacific theater of the war were under pretty much the same orders (very little was publicized about the submarine warfare in the Pacific until well after the war).
Submarine warfare is particularly nasty. In most cases the crews of ships sunk would stand very little chance to be rescued. Other ships in a convoy would not stop to pick up survivors for fear of being attacked themselves, for the same reason (and the fact that there is very little room on them) submarines would usually not make any attempts to pick up survivors, and escort ships too were first and foremost out to attack the submarines rather than taking the risk of stopping and picking up survivors. So in most cases no serious rescue attempts were undertaken to begin with. And also much of the submarine warfare took place in the North Atlantic where the icy water would kill most of the unlucky sailors unlike in case of the Laconia which was sunk in comparatively warm waters of the African shore.
In addition to the order not to safe enemy sailors, there were also cases in which sailors of sunk or sinking ships were actually shot by the crews of German, Japanese, and American submarines. Submarine commander Heinz-Wilhelm Eck was the only submarine commander tried for war crimes (though to be sure he was not the only German submarine commander to have committed this kind of crimes). He had shot survivors of the Greek freighter Peleus and was executed for this in 1945.
However the same kind of crimes were committed by allied submarine commanders as well (which does not mitigate the horror of atrocities committed by one side but rather increases the horror of the overall picture). US submarine commander Dudley W. Morton for example shot Japanese sailors of ships he had sunk without facing any legal consequences from this though it has been speculated that it was for these actions that he did not get the medal of honor (on the other hand he is held in proud memory as a hero of the Pacific war and a destroyer of the US navy was named after him). The same kind of action (machine gunning sailors drifting in the water) did not prevent British submarine commander Anthony Miers from getting the Victoria Cross though. In submarine warfare there are few examples of the participants shining for humane treatment of the victims
Another example of atrocities against defenseless enemy combatants would be the practice of shooting enemy airmen after they bailed out of damaged planes and were dangling on parachutes. But I guess this would be off topic unless we intent to widen the scope of this thread.