You may find this Tripawd Talk Radio podcast helpful for learning more about swim therapy and when it is and is not appropriate:

Dr. Jessical Waldman Discusses Canine Rehabilitation Therapy

From what I've learned, swimming is terrific exercise for Tripawds who have gotten the pain under control and demonstrated more joint flexibility, but new Tripawds should hold off on the swimming until they've reached that same level.

“Hydrotherapy is nice for gait training, active range of motion of certain joints and it's nice for strengthening,” says Dr. Jessica Waldman of California Animal Rehabilitation. “I think it's a good tool when used at the right time, but I wouldn't put an amputee post-op in any kind of water for at least four weeks until I felt like pain was under really good control.”

Hope this helps!