I posted a similar thread a couple of years ago to get information and to stimulate some discussion.
I have triplets (2 DS and 1 DD), and my 2 sons have cerebral palsy. While we haven't been to the water parks with our kids yet, my wife and I did get to go to Orlando for a business trip without the kids, and we went to Typhoon Lagoon. While we were there, we ran into a family with twin daughters, both in WC with CP. We tried to help them through their day.
The park is not a "wild" water park in that kids aren't running through rampant knocking other kids down. Those twins had some trouble with the wc's, and we helped navigate. The river was wonderful, and I think that would be a nice activity for our kids. I know some people have stayed away from the wave pool, but if you have some balance, can stay toward the back (toward the "beach"), and have someone to help the disabled person (buddy swim), the wave pool can be fun. I think our boys (with their floaties or vests) could enjoy this.
The slides are the biggest obstacle. I did help carry those 2 girls up the hills for a couple of slides. And they had a blast! And I was exhausted! We may try that with our kids before they get TOO big (and too heavy for me to carry). I know that BB has a "ski lift", but I think that only leads to that HUGE drop slide, which I don't think my kids should be on.
By definition, a water slide park is difficult to make accessible. But I think this is one of the last frontiers for accessibility for WDW, which by and large, does such a great job for disabled guests.
Wade