As I have mentioned in a separate post, I have a bad back brought on by a congenital condition (my L-5 never fused correctly, so I have a crispy shell with a gooey center). For years, I've been trying to find the Silver Bullet exercise or stretch.
I believe I have finally found it: Tread water for 10 minutes as many times a week as you can.
Honestly, the results have been tremendous and incredible. Not only has it helped strengthen my back muscles, but also it has helped strengthen the rest of my core, arms, legs, and (this surprised me) neck. And obviously it is as low-impact an exercise as you are ever going to find.
Anyway, just thought I would pass it on. I'm able to go to the gym by my office and get in a "complete" workout in about 30 minutes.
If you've got back pain and are looking for something new, I can't recommend this highly enough.