They are dead either way so it's not a moral thing imo but there are some things I go by in hunting, ducks are hard to kill on the water in my experience, I've seen cripples shot with 12 ga loads multiple times by multiple shooters and still get away. Sometimes that happens and sometimes 1 shot will do, but I don't water swat dux cause I just don't.
Things I go by are no water swatting dux, no shooting squirrels the dogs haven't treed and no shooting turkeys unless they have been called up plus I don't kill anything on the last day of any season when I go on the last day, not that it makes any difference but I figure if a duck, squirrel or whatever has run the gauntlet of hunting season, weather, natural predators and sickness and has made it that far I tip my hat to them and wish them well in reproducing in the spring.