For people with joint and back problems, aqua-workouts are ideal - the water acts as a giant cushion for the joints, protecting them from the kind of stress and impact they experience when exercising out of the water. Even people who can't swim can workout in the pool.
I have a temporary disability (knees) that keeps me from effective exercise on land, but with aquacise, I get a good workout with minimal pain. I'm not a good swimmer, so I wear a flotation belt and "swim" laps, jog, do stretching exercises, leg raises, ab crunches, knee flexing, upperbody workout, and so much more.
Most exercises can be done in the shallow end of the pool if you prefer.
You can increase resistance by wearing ankle/wrist weights.
Here are some free videos on basic aqua-fitness pool workouts.
Note: Always check with your doctor or physical therapist to see which exercises are best for you.
You might find some more information in this thread.