I'll add to the good advice already given.
I would let the duck swim in a tub of lukewarm water so that the wound gets rinsed off, and so that you can examine the duck closely - look around the vent and under the wings for more puncture wounds.
Storey's Guide recommends clipping the feathers a half inch away from the edges of the wound.
You can find a saline solution recipe in WebMD, by the way.
You probably already do, but be sure to talk to the duck like you would an injured person. They are vocal, social creatures and the voice can really soothe them.
I don't know what the weather is like where you are, but if it's warm, there may be flies and you need to keep the duck in, away from flies because they will lay eggs in the wound. yuck, and dangerous.
Please keep us posted.