Windsurfing can be bad for your back but not in the way you'd normally suspect.
We tend to build up quite a bit of muscle in specific areas across chest and back but our core tends to just wobble around a bit (some of us more than others

)
Much as I hate to admit it after the crap I gave Greeny and Windy about it, SUP'ing



has been excellent for helping fix up a few niggles I was having in my back by fixing up my front

I've even been caught out enjoying myself paddling around on it