My GSP is 6. I've done nothing but duck hunt with her. She is the smartest dog I've ever owned, she will run with any lab I've been around. I just rescued another GSP male, he was HW positive and untrained at 2yrs old. Im trying to train him now while I treat him for the HW, he is a project though.
I have a neoprene vest for mine she hunts in boat with me. she does fine with the cold. If its really bad cold I'll put a heater in boat blind for her. But it doesn't get that cold down here. Just make sure you can get them out of the water and wind as much as you can.
They are great dogs and can be very good duck dogs. Mine has over 300 retrieves including geese.