A little bit of swimming will help build up muscle tone without putting stress on the bones. But I wouldn't go for more than 5 minutes or so at a time.
Mel, don't worry too much about your puppy classes. Most puppy classes are structured to allow for plenty of rest between exersizes. While classes do go for around 45 minutes, a lot of the time is spent talking and socialising. If a puppy is looking too tired in one of my classes, I will ask the handler to allow the dog to rest. When training at home, puppies only need 5 minute training sessions. Discuss the issue with the instructors at your school to see what they have to say about it. As you are learning to instruct, it is topics like this that you will find interesting and helpful.
When you enrol in the Delta course, let them know the age of your puppy.
Whether or not the dog, or the breed of dog is prone to hip or elbow problems, the guidelines for not over exersizing still apply. All dogs have growth plates that can be damaged, and the larger the breed, the longer it takes for the bones to fully form.