The point here though is not whether hydrotherapy helps muscles or joints, as that is well established. The issue is whether it can help dogs with SM, amd SM has nothing to do with the health or strength of muscles or joints.
Tania clearly had to stop hydrotherapy with a dog *because* it had SM so I would not think it would ever be considered to be a therapy for SM. But certainly some dogs with SM would not be bothered by swimming. It is the neurological pain caused by stimulation of nerve endings in the spine from body movements or ANY type of increased CSF flow that causes problems for dogs. CSF moves faster with any sort of exercise, so swimming could potentially aggravate SM symptoms for dogs that are sensitive to this. Swimming is much more demanding and tiring than walking and will have a greater effect on speeding up CSF flow -- not very desirable generally. And actually there is also more pressure on the body in water, pressing evenly all over, which could potentially aggravate SM.
The harnesses sometimes used for hydrotherapy could also really aggravate a dog's pain.
Set against that, many dogs with SM enjoy exercise and enjoy swimming. But anytime you increase the flow of CSF you risk increasing the effects of SM and worsening syrinxes (believed to be caused by CSF pressure in the spinal cord). All dogs differ so every individual case has to be considered but I'd not put my SM dogs into a hydrotherapy tank. I do however let them run around and play -- I do think this gives a quality of life that is more important than longevity if it means sitting and doing little.