Interestingly, I had a similar conversation three days ago at the Newfound Woodworkd Rendezvous. A canoeist was looking for a sea kayak to take on down river trips. He was test paddling a Guillemot. The consensus we reached standing on the beach was that the Guillemot was as manueverable as anything around and could probably handle Class II water if you were just interested in passing through. I wouldn't hesitate to take a Guillemot our in ocean surf, but you can surf all day and never hit a rock. On a river rocks are inevitable and there is less room to maneuver than in the ocean.

I'd have to say that if you know you're going to hit rocks a plastic boat is the answer and if you know you'll be in Class III water a sea kayak is probably not the answer. The length of a sea kayak can be a real liability in rough water; the paddler exerts controlling forces from the center of a 17 ft. boat, but the water can act on the ends of the boat, gaining great leverage. A canoe, with paddlers placed at either end, makes more sense as river conditions get more severe. The other alternative is a white water kayak, which is short enough to negate the water's leverage advantage.