Swimming is not the only way to exercise in a pool. The natural resistance of water to your movements helps stimulate your muscle activity and because of the body's natural buoyancy, these exercises put minimal strain on the joints, ligaments and muscles.
To exercise arms and shoulders, stand neck-deep with your arms stretched out to both sides. Bring them together in front of you, keeping them straight as you touch your hands. Then bring them back out to the sides and repeat.
To exercise your legs, try the "rocking horse." With your right leg raised and bent at the knee, push off with the left leg and land on the right. Then push back with the right leg, landing on the left. Repeat the exercise for several repetitions.
Of course, never take water for granted. Familiarize yourself with the depth marking before you enter the pool. Even if you're a good swimmer, it's best to exercise when a lifeguard is on duty.