We (teen DD who uses a manual chair and myself) have been to TL multiple times, but we have never been to BB. There is plenty to do, but most of the slides are NOT accessible. The only accessible slide is Crush n Gusher, where you can take an elevator to the top. But it is not a slide for the faint of heart. I'm a big time ride weenie, but DD LOVED it. There is a height requirement, and I'm also not sure how good it would be for someone with upper body instability.
I would advise getting there at park opening. There are water wheelchairs you can check out, but they don't allow for any special positioning needs. Still, you can take them up to the wave pool, something you might not want to do with a personal chair.
With the wave pool, we would kind of drag DD out of her chair and then we would sit in the shallow end. The bottom of the pool has a rough texture, so I HIGHLY recommend wearing board shorts (the kind surfers might wear; they tend to sell them near bathing suits or at sporting stores). Also, I can't say enough for those aqua sock pool shoes to protect the feet from getting scraped up.
We also did the lazy river. There are bump steps to transfer in, although it's not a graceful transfer into a tube and DD and I looked quite silly trying to get both of ourselves in.
Anyway, it was a fun day, but that's because both of us like just hanging out in the water and sunbathing. Someone in a chair who is expecting to ride slides might be disappointed.
Mary