There are many benefits to exercising in the pool, some of which you might not be aware of. On the one hand buoyancy eliminates up to 90% of your body weight, making all water exercises low impact and safer on your joints, especially if you are coming off of an injury. On the other hand since water offers up a lot more resistance than air, simply moving around in a pool or lake, even if you are not moving fast, taxes your muscles in a good way. Whereas walking on land only really engages your lower body, walking in a pool where you are almost submerged works everything, including your core.
Another benefit is the impact of hydrostatic pressure, which is the equal amount of pressure water exerts all over your body. This aids your circulation and helps keep your blood pressure down. Consequently your heart has to work less to do more. If you are someone whose lack of cardio-respiratory health prevents you from exercising more, a pool workout might be your best chance at working your joints and muscles without getting too strained. Hydrostatic pressure also helps your general circulation and brings other benefits, like your blood cleaning out toxins faster and reduction in edema.
Let's not forget swimming and its many benefits. Your muscles have to work hard to propel you through the water while your joints have a reduced chance of injury. Since your lungs have to work a bit harder to expand against the surrounding water pressure you also improve your lung capacity. So if you are a regular runner or cyclist you might consider periodic swimming to improve your endurance and to shock your body.