Donell,
How bad is the displasia? I know MANY guys that hunt dogs that still have displasia, you have to be very smart in how you work them though. Do not let them jump in and out of the truck, this is hard on them and probably hurts them more than hunting. Swimming is easy on a dog, its all the running around and stuff thats hard on them. If your dog swims well and the retrieves are short, I would still take him out, keep him on a leash, don't let them run around. Lift him from the truck to the ground, when the hunt is over, lift the dog back into the truck etc.
Some may disagree, but think about it. My dad has severe rumatoid arthritis. It kills him to fish, but he does it every chance. Because it makes him happy. When a dog loves to hunt, make some modifications to your style so they can still hunt.
Oh, and if the displasia is really bad ignore this post...
Good luck man, displasia sucks and it is really common in labs. I have seen some pricey dogs from great breeders come with displasia.
Mike