The boats are on tracks. If the CMs are driving, if you notice, alot of times they're not looking where thir going when they steer, and the boat always goes the right way, so thats one give away. When I've seen the guide let the guests drive, it's always been a little kid so they won't know that the boat's not going where their steering it. Do you think Disney would let a tourist rash a boat into an expensive animatronic animal?
I'm not sure about this, but I think the "drivers" on Jungle cruise have the ability to control when the boats go foward and backwards, and how fast. It seems like sometimes they spend a lot of time at that water fall! I know on Jaws at Universal though, which is a simular ride, the drivers have no control of the boats what so ever, and they move automatically, and are started up at the loading dock.
Hope this helped.