That was very controversial in the First World War. Submarines didn't make their presence and demands known in the way that surface ships couldn't help doing.
P.S. The artist Edouard Manet was on the coast when the battle took place and rendered it in this painting.
Germans used warships and converted merchant ships as surface commerce raiders during WWI. They even used a 3 masted sailing ship for one. The Seeadler captured 15 ships before grounding on a reef ended her career. They used warships such as Graf Spee and converted merchant ships such as Atlantis during the WWII. By the time WWII rolled around, the controversy over using submarines to interdict enemy shipping had all but disappeared. The CSS Alabama was well within it rights to attack and sink U.S. flagged commercial vessels.