I don't know about GWPs for ducks, but I have some first-hand experience hunting Drahthaars for ducks.
Drahthaars will break ice! All three of my dogs would break ice and just swim around, even before I started using a neoprene vest. Now, I use neoprene because there is enough research to convince me that extreme conditions shorten a dog's life.
There are some Drahts that have better than average prey drive that makes them better waterfowl dogs, but all can do the job. And, short of extremes in temperature (they don't have the body mass of the big labs) and dangerous tides or currents they can hold their own with any hunting dog.
A study at Auburn U. placed them second in nose to the bloodhound, so they really shine in thick marsh and swamps where cripples quickly are out of sight.
GWPs out of NAVHDA breedings and Drahthaars (implicitly) are tested for nose and the tracking ability that makes these dogs invaluable.
The real plus is that these dogs, properly bred are as useful in the water as the upland and serve the purpose as bird dogs on pheasant, grouse, and quail. And, their moderate size ~65# allows them to function well in the home.
Hope this debunks any wives' tales and helps your decision.
Regards,
Bob