I'm 6'3" and completely screwed up my lower back from rowing. In 2013 I had got to the point where I could not sit down at a computer for more than 10 minutes.
After four years of swimming 3-4x/wk, my back is now totally fine! Highly recommended.
Swimming is definitely something I've thought about, especially because I want to be good to my knees as well - years of outdoor basketball took its toll haha. My problem is that I find it pretty boring, so I have a hard time getting motivated. I should just do it anyway, or make some time to go to the beach or something.
After four years of swimming 3-4x/wk, my back is now totally fine! Highly recommended.