Extremely dangerous on ANY moving water. you won't find a manufacturer anywhere that recommends their float tubes for moving water... I know guys do it at the mouth og the Deschutes but umm well thats just dangerous and i think you'd find that many a coast guard patrol would tell you the same thing.
I don't care how many pfd's you wear and how careful you are it's extremely dangerous just get a pontoon boat that keeps your legs out of the water!