As the Fish and Wildlife Service is silent on the matter, it is a state decision as to whether the practice of shooting waterfowl on the water is legal or not.
The early game laws, especially waterfowl, were based on rules adopted from the more prominent duck hunting clubs of that state. Some clubs condemned such practices while others were silent or it was condoned under certain conditions. This is a significant factor to the hodgepodge of legalities on the matter.
As for ethics, I am not a fan of shooting ducks and geese off the water. It has more to do with the difficulty of killing them rather than the ethics of merely shooting them while sitting. A duck or goose sitting on the water has much of its vitals covered with multiple layers of feathers and bone which makes it difficult for pellets to penetrate. Hitting the head/neck is possible but is also difficult as larger pellets often lack the pattern density to hit such a small target while smaller pellets may run out of energy to consistently complete the task at moderate to long ranges.
Ive been on both side of the issue over the years and at different times in my life. I've based the above opinion on the numerous times I have sculler for ducks and geese, a few times where I shot them sitting whether on land or water, and the times I've had to finish off a cripple. I just prefer to shoot them flying as the vitals are more exposed than when sitting on the land or water. I also tend to find waterfowl are less bunched when in the air which makes for fewer "Dutch doubles". This is especially true when sculling as the birds tend to crowd together when they become concerned by the object drifting up to them. It is a trait market Hunter's took advantage of as it maximized the number of birds bagged with one shot.
Though I have a personal ethical issue towards shooting sitting waterfowl, I don't begrudge others for doing so if it is legal and safe. For some that is the most ethical means of shooting as their wing shooting skills are rather poor. For the often limited times they get out and the even lower percentage of success, I don't see them having much of an impact.
