Hydrotherapy is exercises done in a heated pool. The water helps provide support, and the warmth also makes a difference to painful joints. I find hydro exhausting, but it does help ease joint pain, and to loosen up stiff joints a little.
To get my hydrotherapy program, I went to a physiotherapist to have exercises tailored to my needs - and I'd recommend anyone else to do the same.
To give you an idea of the kinds of exercises involved, here are some demonstrations from Gary Moller on You Tube.