A companion or a retriever? My Dad and myself trained 3 golden retrievers for duck hunting. And they were great dogs (though one was gun shy), I'm sure a boxer could be a companion but having down my fair share of duck hunting it's safe to say the boxers temperment isn't so well suited to be quiet in a duck blind for hours. Also I don't know how your dog swims, my girl loves water and wishes she could swim better but the bottom line is she's 55lb of the leanest muscle you'll find which makes her more like a lead weight than a fish. One thing you probably won't have to worry about is them being gun shy. Over 4th of July we lit up a bunch of M-80's and M-100's and my girl never flinched. I would say if you're going to do it start as young as you can. I'm not saying it would be impossible just a challenge, good luck keep us posted and have fun.
Garnet 6yr old Doberman docked/floppy, Ret. seeing eye dog
Mercedes 4.5yr old Boxer docked/floppy, certified therapy dog
Bentley 18 months Boxer cropped/docked, working on agility training