Aquatic therapy classes can be an enjoyable way to get aerobic exercise, and it's easy on the joints. The water's built in resistance increases strength and endurance. Many people find it very relaxing--especially just taking some time to rest, breathe deeply, and float. If walking on land creates weakness in the legs, striding back and forth across a pool can be the perfect strategy to increase heart and respiration rate, without the risk of falling. For those who want to try conventional swimming, the backstroke requires less extension in the lower back, and can be a little safer for stenosis.
Advantages: 1) Reduces compressive forces on the spine. 2) Can be tailored to fit one's individual fitness and strength level. 3) For those carrying some extra weight, it can allow a feeling of weightlessness and greater freedom of movement. 4) Community pool programs usually foster a fun, motivating social environment. 5) This is an opportunity to exercise safely under the supervision of an instructor.
Precautions: Because of the water's support, first-day enthusiasts sometimes do very large movements in the pool (it's easy and can feel terrific at the time), then wind up feeling sore and aggravated later. Avoid this pitfall by keeping yourself to about 25% of the normal range of movement you'd perform on land--especially for your very first class. If your instructor tells you to lift your leg as high as you can, lift your leg about 25% as high instead. See how you feel over the next 2-3 days before you try any bigger movements (especially trunk twisting and leg lifts) in the pool.