If my pup of 15+ years was alive and cancer-free, I would've rigged a baby-carrier style backpack for her. She wouldn't have been able to walk much in her old age.

They have these neat life vest sets that you and your dog can wear, with loops to rig a leash. A guy I know goes canoeing with his lab, but the dog will get tired of swimming and needs to get towed back in the canoe after a while.