I row from ethafoam covered drybox. The ethafoam is miserable on its own, so it is always topped with something. I've tried a "tush cush" from Cascade River Gear (think mini paco pad), but I always end up sitting in a pool of water. I've also tried a Crazy Creek chair, which was kind of meh. I have a couple of Aire IKs so I've used the Cheetah Chairs out of those a few times, and that has probably been the best solution for having a backrest, but they are kind of a hassle to strap down and I don't like stuff that can potentially catch my oar handles at crucial times, so I moved on from those as well. In the end, I almost always end up just using a cheap square-shaped throwable boat flotation cushion and use the gear pile as a backrest. It gets me up off the the uncomfortable ethafoam, elevates me an inch or two to see over my passengers, keeps me from sitting in a puddle, doesn't soak up water and become soggy, and since it is attached only by a strap through one of its loop handles at the back hinge side, it is simple to flip out of the way if I need to access something in the box.
I've been chasing the perfect solution for years, but I'd guess that cheap little cushion has been my go-to for at least 5 seasons now. You can get them in a variety of color schemes to match your boat too: