I've been giving some thought to this lately. I, like most, have been told for years that only flying fowl are legitimate to shoot at. I'm starting to question that though. The rationale behind it is mostly one of "fair chase" but shooting birds on the wing results in huge rates of wounded or recovered birds.
This source suggests that rate is as high as 25%, including many that hunters believe they missed cleanly, or that took a stray pellet from a shot that downed another bird. I know most will claim their rates are much lower, but anyone who says they dont lose an occasional duck is a liar. In my view, the ethical imperative is to kill the animal cleanly and with the greatest chance of recovery, not to shoot it in the most challenging way. Wounding and losing a bird seems the greater wrong than shooting a stationary one. That seems to be the case in deer and Turkey hunting, where shooting a moving target is more frowned upon.

As a side benefit, the breasts and legs are below the water line, and dont get nearly as much shot damage as a bird shot on the wing.

I realize that ricochet risk is a very real safety concern in some areas, but where it isn't, why shouldn't one shoot sitting ducks?