Salt water brining really depends, IMHO. Is the duck a diver and likely to be fishy? Then brine it. Also brine it if you are smoking the duck or if you shot the heck out of it. Salt water cleans the shot-through spots.
But, if your duck is a rice-eatin' mallard or pintail or teal or wood duck, DON'T brine it. These ducks are best eaten simple, and the juices inside are part of the treat.
This is my take on it, but I like my ducks to taste "ducky." If you are serving to people who might wrinkle their noses at anything remotely wild-tasting, go ahead and brine it.
I love all of God's creatures, properly prepared.