There is nothign illegal about that at all. In fact, on slow days when I am by myself, I will try and land a flock of more than 4 birds in order to get three quality shots off- giving me a for sure on the first bird and then a good chance to finish off the triple..that is just me though
This year I had a redhead that skirted the decoys and landed at about 35 yards...I did all I could to get him to go airborn but he just start swimming hastily- needless to say I wasn't going to let him get away- and put a great 40 or so yard water swat on him.
The only problem is that by water swatting you are aiming for a much smaller area with very little vital body parts in a good position to make a kill. Obviously the head/neck is the best place to hit- but hitting any other part of the bird and you will have an almost for sure cripple.