WELL, we now skip the water parks..... but we did them for years, and we all loved them. Even DS, who has the special needs. We have been to both water parks many times. DS is now 20.
When DS was younger, it was not too bad. DH would carry him piggyback up the mountain. He has Down syndrome, and Autism, and could walk. (but he did have orthopedic issues, and had the AFO braces when he was younger, eventually had to have his ankles both fused, low muscle tone, etc). As he got older, this got harder. He has balance problems, and his vision is off, so while going upstairs is slow, going downstairs is REALLY slow. Some of the trails up the mountains, include some stairs going down, as you go up. He is very slow walking up the mountains, and people of course pass us constantly (understandable).
We did go to Blizzard Beach many times, thinking at least they have that lift. Here is the thing, at least years ago, they do NOT honor the GAC at the water parks. They had no real policy. We had a family of 6. We did not ALL try to get on the lift thru the handicapped entrance. The lines get long for that lift. We would have 4 of us walk up, and DH would take DS to the lift, we would meet at the top. Sometimes the 4 walking up had to carry extra tubes, because some of the slides require that. We had one outrageous incident where the CM denied them the handicapped lift entrance, saying DS "looked fine". (he was reported and reassigned that day). Then on another trip, the CMs told us DS had to use the special chair that is like a gondola, because of the autism. (that is ok, i can see the reasons i guess). OK, here is another thing. Sean uses his chair at the parks, so we would have him in the chair, go to the lift. We would have to leave the chair somewhere, and the slides end up all over the place. One family member would have to sprint over and bring it back to the exit area of whatever slide we took. At the top of the mountain, there were long lines, they would provide a wheelchair, at the top, to take him to the slides, but as we got to the front of the slide line, a family member had to run the chair back to the top of the lift area, so they would have it for someone else. They did not have a CM to do this, so as we got close to the front of the line, we would run the chair back, and it created some havoc.
Bottom line, it got to be increasingly hard to deal with..... we did have a good working plan though. We have 4 kids, and we would arrive early, at water park opening. We would all ride the family raft ride together. Then DD would take DS on a few more slides, then go to the lazy river. Meanwhile, after the raft ride, Mom would ride the big slides with the other 3 kids. We usually all did a lazy river loop together either before or after lunch. We would meet for lunch, and afterwards, Mom would usually leave with DS. He would be wiped by then, and Mom already rode the slides in the morning. Dad would then stay for several hours, with the other 3 kids and ride the big slides. This worked pretty well for a few years.
It is possible things are easier/better now. In my opinion, they were not designed and built to accommodate people special needs. And it would be hard to think of a way to do this any better. We put Sean on the pool slides, he likes those....... we will be at BW in a few weeks, and he likes that clown slide.