My (now 14 year old) small mixed breed started Hydrotherapy in September 2019, it's really helped him. He moves so well on his walks no one can tell he's elderly. We built this up going weekly to begin with then dropped down, in the summer he only needed 1 session a month. Over winter he goes every other week generally. His hydrotherapy centre also sell and recommend Golden Paste, I couldn't try it for Jack as in was contraindicated in the case of his Calcium Oxalate and Struvite bladder stones unfortunately.
They also sell Joint Aid, which Jack was also on alongside his veterinary prescribed Synoquin. We haven't built his supplements back up yet after an acute pancreatitis episode (as you may know from his other thread) and haven't noticed him struggling yet since he's been swimming recently.