In the migratory act it used to state, and it still does in some state regs that you cannot shoot a bird "at rest" So its sortof up to the COs discretion. Is a duck swimming around considered "at rest" yes and no, so its a very grey law.
Like most migratory bird act laws they all stem from the days of market gunners. And the whole not shooting ducks "at rest" was to stop guys from sculling into a whole flock of ducks and shooting them with punt guns all at once.
Shooting em on water, its not unsportsmanlike, its perfered!!