Hi Marty,
I really envy you on this, haveing water flow like that is great !!
I would suggest, you enclose the vane, kind of like a heater fan on a car, but open at both ends on the down stream side only, so water flows down the active side of the blades. and back out -- perhaps you could add a couple foot long tube, to the incomeing side,angled down into the sream, and mount the whole thing vertically ,above the covered area over the stream -- this would keep water off your gen, and prevent emersion !!
Lots of work, but not that difficult -- (easy for me to say) Looks to me like what you have is going to be worth while -- And a brushless gen, in the future as you will be surprised how quickly those brushes go away, at 24 -- 7 of spinning !!
Nice, Bill H......