When I was a young boy my Dad and I were sitting in a blind when a duck landed in the decoys. As I raise up to shoot my dad tugged on my arm and said "Son you aren't going to shoot that duck while he's swimming are you? and I sad "heck no, I'm going to wait 'till he stops!"

Shooting ducks swimming or flying is a matter of preference. I would certainly never chastise a person for shooting a sitting duck. As long as it is legal it is ok with me.