I sure wouldn't go out without my oars! The prop would get knocked pretty fast in most of the rivers I drift. When you go down fairly fast rivers, you are often forced to "straddle" big rocks. My anchor, which hangs out of the water in the middle of the rear platform takes some heavy knocks.
Also, I have gone to a 9' pontoon because it is less difficult to hold a straight line than with 8' pontoons, even on stillwater. I am skeptical that you can hold a straight line with just the prop. I would like to try one though.