I think you could easily train your gsd to retrieve waterfowl. Focus on soft mouthing or you might end up with some half eaten ducks. the rubber training decoys with the " rubber nubbin spikes" on them are good for keeping the mouth soft on retrieve. Start using soft training decoys and work on the soft mouthed retrieve and you should have a boat buddy in no time. I got my boy midnight soft mouthed on ducks in about six months, but we do have a river, and ponds and creeks right close. Plus i jump at any chance to wander around a wilderness area so that helped.
The only reason I think training a high prey dog drive to point is because the best pointing dogs I've hunted with (none were mine as I have only hunted with others dogs) just point. Most have been breed to just point for a very long time. No uncalled running or flushing, just a stiff legged whole body point that they hold until they hear/see the shots. Sometimes it takes a long time to creep up on those upland birds so the dog may have to point, just out of pouncing range of said bird, for a while.
I know a few people that have them trained to retrieve after the shots. Those pointers I've hunted in packs with also are trained and bred to honor the points of other dogs in the brace, freezing in position when they see a bracemate on point. I'm sure you could train a gsd to do almost anything so pointing (and the more important holding at point) would just take some special methods and lots of effort.