you may want to look at NGC marine electric motors. they can be used as propulsion or regenerative. So if you can find out the way they are built you can possibly make them. On the St-Laurence river (2 miles wide) we have tidal currents downstream 3 to 4 knts, upstream 2 knots. If you have a good anchor system, your raft generator will turn itself depending on the tide direction. Deflectors will accelerate the current, the loads are surprising. If your deflector is snuged against the bottom and going up like a beach, you shouldl have a better result. We see this at the occasionnal old ferry docks, if we swim at the end of the dock, 2 things happen. 1 - a faster current down stream, 2 - a suction back upstream current swirl, you could i suppose use both for more efficiency. The suction is so powerfull it can drag a kid underwater, especially at the 5 knots areas. Do you have environmental issues in your area ? Check it out before you do anything in the river. Here in Canada, you need a permit to look at the water ...
Water is 800 times more powerfull than air i am told. Most likely, the wheel would be your best bet, it is on the surface and algeas should not clog it, Then you can use the volvo break gen design, build a combo wind/water system ...
mike