I looked into that at one point, but IIRC, the challenge was that the alternator was tuned to have the most optimal output when it was running at thousands of RPMs, so it's not just a matter of hooking wings to an alternator.
That's not hard to fix. You need to use some kind of mechanical system to connect the shaft on the water wheel to the Alternator, which means you can use gears.
So put a huge gear on the shaft of the water wheel and a tiny gear on the shaft of the alternator.
What tons of people do to play around with the optimal setup is to use an old bicycle rear tire for the water wheel and the whole chain and gears setup to connect to the alternator. That way you can change gears while it's running and figure out the optimal gear ratio.
I'm chuckling to myself now because I'm thinking that you could do maximum power point tracking automatically if you used one of the electronic derailleur systems like Shimano Di2.
