When I developed a running injury a few years ago I needed a way to retain my fitness, yet rest my knee. Someone suggested I try aquajogging, so I did.
The Aqua Jogging Belt was available to me at our gym's pool. It is a large foam belt you simply strap around your waist, then jump in the water. It keeps you afloat, so you can "run" in the water and while keeping you from sinking.
The fact that it is the most boring form of exercise notwithstanding, the belt itself worked really well. I got a terrific workout each time I went to the pool. My workouts ranged from 30 to 45 minutes and the belt never slipped or grew uncomfortable. The belt itself is adjustable and wide enough to be comfy.
If you are looking for a great aquatic workout, I would higly recommend using the aquajogging belt.