I was going to ask the same question about "in water or out." If I hadn't asked, I would have assumed in-water, but that's probably because my back used to hurt quite a bit during dives. Ouch!
Mine has gotten better and I think it is from a combination of things.
1) When I was using an ill-fitting BC, the tank would tend to "flop" around and I had to compensate a lot with my muscles - or at least that is how it felt. It just so happened that, for me, a backplate was a good solution for a BC that fit, so for me a backplate did help, I believe. But that's because it fit probably more than the style.
2) Reducing weight that I carried, and changing its placement seemed to help too. I think the jacket BC's I was using were somewhat inherently buoyant, so I had to carry a bit more weight with them. But even after going with the backplate, reducing weight continued to help (as I got better at not being overweighted). But perhaps having a good bit of the weight flat on my back was part of it, with the SS plate --- I'm not sure about that.
3) Posture. I think a biggie for me is that I was arching my back. About halfway through a dive I would just want to curl up in a ball "forward" to relieve my aching back. To help that, I concentrated on keeping a straight line from my chest *past* my back, and then only "arching" below that (in other words not arching my back). That helped a lot I think.
It seems to have gotten better gradually, so I'm not sure how much of it was gear-related and how much was developing better posture/muscles (which is why I kept saying "seemed" and "I think." I do think that any BC that lets the tank shift around has to make it worse though, if that is happening.
Probably head-heavy trim and have to arch back too much. BPW may help with trim, or may not. Need to identify what the root causes are first.
Oh, I just read this. You know, I
was quite head heavy at first, very consistently, so maybe changing that is a good part of what helped me. After I got my own BC that fit, I was then able to start playing with things in order to improve my trim. I can now make it so I'm not head heavy and maybe that is a big part of what helped me. And as you say, figuring out the specific cause/problem is key.
Blue Sparkle