I did see a documentary on a U-boat captain that spotted a lone freighter in the atlantic in the middle of winter. He torpedoed it and when he saw the sailors on the lifeboats and on the water, he knew they would not survive more than a few minutes. Since it was overcast and almost dark, his u-boat actually went to rescue the survivors.
Another U-boat captain used to prowl the florida waters, he would surface and use the deck gun to sink ships in plain daylight. Funny thing is, he would position his u-boat between the beach (full of people) and the target, so as to not have his deck gun miss and kill the people in the beach. FYI, putting your sub in shallow waters and between a landmass and a potentially hostile surface vessel is very stupid (very dangerous for the sub).