I have just started my horse on a bi-weekly regime of using a water treadmill. Due to his PSSM we have had trouble getting his hind end muscles as developed as the rest of him. My vet recommended it (or I would not be doing it), but I do not know of others who have used water treadmills to develop muscles.
The people at the sport horse rehab place say that, once muscles are developed to the point that the horse can use them effectively, there is usually no relapse into hind end laxity.
I would love to hear stories from others who have been through this experience.