Years back, I had chronic low-back trouble with occasional acute episodes (transcendent pain, numbness in left leg, etc.) and was really worried about my first few multi-day river trips.
The boat I chose was a Jack's Pack Cat, mostly because Jack and I are amigos, but also because it was really kind to my back. The seating position is similar to a kayak, but the back support is a high-pressure inflatable pillow that can be micro-adjusted both for fullness and fore-and-aft. It's really a great back support. The seat and footbar are adjustable, so the bending angle of your legs can be changed easily, even while you're floating (not the case with most kayaks).
Quite helpful is being able to push against the footbar to keep your back firm against the rear pillow, which is a subtle but excellent bit of exercise.
In any event, I came off my first multi-day paddle trip (Deso) with my back in better shape than at the put-in.
I really love Jack's boats and the bad-back thing is a big reason. For the Grand Canyon (and given a bunch of friends with big support cats) I used a Fat Pack for most of the trip. No back attacks whatsoever.
(photo by Jack Kloepfer)
The photo shows the upright seating position and the comfortable paddling posture. Plus, I'm on top of the boat— cool.