Said underwater turbine is already present: the prop. As a bonus, it can reduce rigging loads at hull speed by dissipating some energy.

Drag via the prop is nowhere going to reduce load at all. The resistence is the same and the force is square of the speed and surface area. So some drag ain’t doing anything. Even in a dangerous sea a drogue and lines would help, but way more than the surface area of a slowly or free spinning prop would offer.

Hybrid sailboats are already a thing. An electric motor/generator sits in line with the traditional diesel engine and can a) drive the prop shaft, b) harvest power when the engine is turning the prop shaft, c) generate power by running the engine with the prop shaft clutch disengaged, or d) (the one you’re getting at) passively generate power when under sail by disengaging the clutch from the engine side and letting the prop spin the generator.

Sailboat props are often variable pitch to allow for better efficiency (i.e. they can be turned completely parallel with the flow of water when not in use), which also allows them to be set at an optimal angle for power generation (with minor speed losses, obviously).

H&H catamarans has some nice diagrams on how theirs work: https://www.hhcatamarans.com/ecodrive

Ah, cool, those are great diagrams!

I first heard about this system design from the Tally Ho rebuild: https://youtu.be/xohxmwPfctg?si=RoJV2za9ZgPldrJJ&t=1497