teh biggest issue is that you're going to get a lot of drag... while a impeller or prop not connected to anything will spin fairly freely, once you add an alternator/generator on that and put a load, it's going to act has a brake. Even wonder why an alternator belt will squeal under a heavy load, like after crnaking where the battery is pulling a lot of power from the alternator?
so, you're better off conecting an alternator to the engine itself than loosing a few knots to drag...
Regenration works on sailboats but doesn't produce a lot of power. Lagoon has been selling a Diesel Electric 42' cat for a few years now, seems to work but the electrical needs are pretty low... when undersail, the props spin the shaft and the motors become alternator. I don't know the numbers, but i doubt they produce a lot of power.