> that by WWII standards, would have been seen as a cowardly violation of the rules of war

Source? Torpedoing anything with the enemy flag, down to civilian boats and merchant marines, was normalized by centuries of precedent by WWII.

  “failing to do everything possible to rescue those aboard is certainly a war crime,” as the Second Geneva Convention requires militaries to take all possible measures to search for and collect the shipwrecked, wounded, and sick.