Swimming helps improve endurance and flexibility, and it's a very beneficial low-impact exercise for seniors. Because the water relieves stress on your bones and joints, swimming carries a lower risk of injury than many other endurance exercises, and it conditions your whole body as you move through the water. Swimming can even help post-menopausal women avoid bone loss.
When you swim laps in the pool, you're simultaneously stretching and strengthening the muscles in your back, arms, legs and shoulders. Trying out different strokes can help keep your routine fun while also working out different muscle groups.
When you're swimming, it's easy to forget about staying hydrated, but working out in water doesn't mean you can get away with drinking less. Make sure you drink plenty of water before and after swimming laps.
If you don't have access to a neighborhood pool, you can look into joining the local gym or YMCA. New to swimming? You might look into hiring a trainer or swimming coach to get you started with common strokes and some stretches to help you cool down after your workout.