The Feds only restriction on migratory birds is to what shot has to be used, no baiting, and the requirement to purchase a duck stamp. Actual hunting regulations are on a state by state basis which also have shot restrictions, limits as suggested by the Feds, etc. I have never seen any regulations in any state that make it illegal to shoot a bird on the water or ground like you stated. There may be, but I'm not aware of any. Most don't call it sporting or ethical, but hardly illegal. This link is a short perspective on just what you mentioned regarding shooting a duck or turkey and I found it quite interesting.

http://www.fieldandstream.com/blogs/...ng-ducks-water