the Aqua classes I go to are also mostly elderly people or people with joint problems, but the woman that teaches is fantastic.
It can be excellent if it there is constant & varying movement, and the water makes it very low impact so you can really move and keep up. I am very overweight, & have no trouble keeping up with the class. And for me a good guideline to a great workout is when I try to get myself OUT of the water - it will be difficult!
I talked my son into doing the class with me, he's 18, a team swimmer so, he's in excellent shape. He kept up quite well, and admitted it was a good workout, although he did have problems figuring out the routines. I think most people avoid it simply because it is a class specifically for people with joint problems, etc. To them, it just spells "easy". They haven't tried it. It's much like the football players telling the dance girls that their workout isn't a real workout. JMO.