Since the Dechutes down in Oregon has been no fishing from a floating device forever I would think WDFW has a pretty good precedent that a rule like that doesn't run afoul of the ADA.

Although it looks like the Deshutes does have this provision:

If a customer is disabled and cannot wade, a special permit may be obtained through Oregon Department of Fish and Wildlife which allows one to fish from a boat that is being held in place by a fly fishing Guide while wading. Disabled permits take time to procure with a medical doctor’s proof of disablement