We took our lovely elderly greyhound bitch to hydro as part of the management of her arthritis. We did it after a lot of discussion with the vet, and with their encouragement. Initially, she went once weekly and after an initial intensive phase, reduced her frequency to a maintenance programme of visiting fortnightly. We think it gave her an extra year of good quality of life and mobility - she passed away a year ago this week of an unrelated problem, age 13.
I would have no hesitation in recommending it, especially if your vet has suggested it.
We had an excellent therapist - very thorough, very professional and very gentle with our dog. She was not a water loving dog before her treatment but used to really enjoy going.
It is expensive, as insurance did not cover us since her arthritis was a pre-existing condition, but your policy might cover you. In any case, we think it was worth every penny.