I've done hydrotherapy with my two DM dogs; I believe it was quite helpful in maintaining muscle strength as the disease progressed. I would certainly consider it again should the need arise with any of the other mutts.
We did it at Kynofit in Effretikon: http://www.kynofit.ch/
I see that they now offer a swimming pool - when we did it (8 and 5 years ago) they only had the underwater treadmill. One of my collies didn't like the treadmill, he tended to panic in enclosed spaces. Because of that, after the first course of sessions I decided to continue with the therapy plan on my own, swimming in the lake. (My guy's behavioral issues meant that he was not the easiest of patients.) I think if we had been able to do more desensitization work he would have been more receptive of the treadmill.
But my other guy had no problems with the treadmill, enjoyed his hydro sessions very much. Now that they have a pool all the better.
If you are doing this as surgical after-care, be sure that you are proceeding under the direction of your vet or veterinary physiotherapist.
Hydrotherapy (or plain old swimming) is a great way to give the dog the exercise he needs without stressing joints/muscles, or to help a dog with limited mobility. As long as your vet agrees, why not give it a try?
All the best to your Alf.
(Do you have a physiotherapist? I know a very good one in the ZH/ZG area...)