Swimming is a great form of exercise. Traditionally it's typical to hear people boast of the "whole body" aspect of swimming--the way it utilizes every part of your physique, gets every muscle involved, and that's true, not just rumor.
The other excellent aspect of swimming is that there is no harsh, hard-hitting impact on joints and connective tissue. Unlike running or even walking, swimming provides a cushion-like environment for your joints, sometimes even easing arthritis pain and stiffness. In fact, many people find the water so relaxing that simply being immersed in water is in itself a tremendous stress reliever, adding additional health benefits to water exercise.
Swimming as a cardiovascular and aerobic workout is a great way to strengthen the heart and lungs, to build stamina and endurance, to tone muscles and even lose weight. Be advised though, that in water, the "sweat" factor is different; you are not going to leave the pool drenched in sweat, but drenched in water. And since your body temperature will constantly be cooling, you are not going to temporarily drop weight through sweating in the same way that you would with other cardio exercise.
Swimming is wonderful for overweight people who may not want to strain the connective tissue of their bodies with higher impact aerobic exercise. The water provides a nice buoyancy that can inspire, rather than defeat those with weight issues to exercise more. Also, pregnant women find that swimming is less draining and more satisfying overall and are often greatly comforted in the water as they continue to maintain their health during the precious time of carrying their child (or multiples!).
As with any exercise, consult with your physician before beginning swimming regularly. Make sure you are a strong, capable swimmer, and, if not, local pools often have lessons no matter what your age.
Kickboards and foam noodles are great in the water and can help you reach your fitness goals. Water aerobics and underwater running are other, sometimes more creative options to stay fit in the water.
You can enjoy swimming all year 'round, no matter what climate you live in.