I come from boats where we wanted to SAIL, and while we made the proper obeisances and blessings over "Our father who art in Evinrude" and left offerings at Herr Doctor Professor Diesel's gravesite, I don't think I've ever spent any long time on any boat with a 3-bladed prop unless it was a motorboat! So, whether a 3-blade should be locked or roll just doesn't matter much to me.
OTOH there's always been a mark on the prop shaft, so the folder can be folded properly, or the fixed 2-blade can be rotated vertical and set to minimize drag. I can't see dragging the water wheel as a routine procedure.
With one horsepower equal to about 750 watts, I'm told you can figure about 25A @ 14.4 per horsepower, allowing for a 50% loss. I don't swear by that, it is just what I've been told. So, a 50A charging rate from a water wheel would be like a two horsepower "drag" on the boat. Maybe no problem for a cat scudding along at 18 knots but for those of us who think moving from 5.1 knots to 6.3 knots is a Really Good Trick?
Dunno. That would take some getting used to.
I suspect the real answer and real numbers with whether to lock a 3-bladed prop are going to depend on a lot of particular design issues. Whether the prop is blocked by flow from the keel, is in an aperature, has other turbulence from the hull, etc. etc. If I have to spend time living with one, then I'll chase down the latest opinions on it, and see if they've changed over time. Or, lock and unlock the prop, and see what Mr. GPS has to say about it.