For what it's worth, the official Disney website has a Guidebook for Guests with Disabilities for:
Typhoon Lagoon
and for
Blizzard Beach
They don't give a lot of information, but it's better than nothing. The Passporter Book of WDW for Your Special Needs does have a section about the water parks, so you might want to peak at that if you see it in a book store.
Blizzard Beach opened in 1995. Typhoon Lagoon opened in 1989. Usually, the newer area is going to be a bit more accessible, since they do use knowledge about what might have worked better to improve the newer park.
The Guidebook for Blizzard Beach does note that many of the attractions are not accessible for people in wheelchairs because they have stairs and no elevators. As far as I know (it has been a long time since our last water park visit and DD did not do any slides), someone in your party is responsible for getting the wheelchair and ride tubes from place to place.
Also, a hint from our experience, if on the Lazy River, pay close attention to where you left your wheelchair to get into the river. It's easy to loose track as you are going around and if you pass your 'stop', the river is only going in one direction and you will end up making another trip around.