If you want to strengthen the muscles around your knees, while protecting your knees at the same time, try doing sets of squats in the pool. Water provides a protective cushion while countering the effects of gravity. After you jump in and warm up a bit, stand in chest-deep water about two feet from the pool's side, facing it. Hold the pool's edge with both hands, and slowly bend your knees and lower into a squat, pushing your buttocks behind you like you're sitting in a chair. Squat until your chin hits the water, then slowly rise and repeat. Do as many reps and sets as you would on land.