It's low-impact because the human body is only slightly less dense than water, which allows the water to support a person's weight
I love to swim and it keeps me in shape Nice!  It's a good "low-impact" sport, compared to running. Yes and I have lung disease its very easy for me to exercise in water, much easier than a gym workout Oh I see!  And swimming is also useful for survival, right? Yes and its also low impact on my bones and joints. Older people really benefit from swimming
Since the human body is only slightly less dense than water, water supports the weight of the body during swimming.