I've read somewhere that Donald Knuth had suffered back pain. His doctor adviced him to swim three or four times a week and he eventually confirmed that swimming is an
effective excercise to cure and avoid backpain.
Makes sense. When I had bad back pain I only really felt comfortable standing while in a pool, thanks to its buoyancy effect. I'd get out and almost immediately have back pain again. Thankfully I don't have that problem anymore.
It can be, or it can be really bad. Depends on how you're swimming. Especially when doing breast stroke, it's very important not to pull your neck back/up. Keep your head down.