I have had no problems dispatching them with #4 2 3/4" across my 1.5 acre pond.
I have a hard time shooting anything on the water (ethically, not the ability). It is pretty fun waiting to see where they will pop up after a dive and still make the shot. I have had them dive in one corner of the pond and come up on the other side of the island which is about 200-250 feet, it makes for good practice.
