Whup! Time out.
Regards any sort of paddlewheel, etc....
As sailors, we understand the difference between a lifting surface
(when your sails are behaving like wings, generating fluid-dynamical forces) and a drag body
(when you are sailing DDW and your working sails are just big old bags, being pushed against by the wind.)
Drag bodies stink for electricity generation. As tschmidty notes above, alternators produce better at higher RPMs, & the fastest part of a drag-body paddlewheel (tips of the blades) can never move faster than the water moving past it. You'll be lucky to get 100 rpms out of it, & the hub will be moving maybe 6/10s of a knot.
However. An axial thrust rotor (looks like a propeller) can achieve tip speeds many, many times the speed of the incoming water. The blades do this just as your sails do on a beam reach, by splitting the flow and generating lift over both surfaces. I can't find data on the Watt&Sea hydro-gen, but I'd bet it is spinning several hundred RPM at cruising speeds.
W&S have basically saved us the trouble of brainstorming the perfect unit -- they've gone & built it. Although ... I'm not a fan generally of downstream turbines. Face the blade to the front for maximum output. I am guessing they face it aft for impact protection, which is a fair compromise.
A trolling motor with a correctly-pitched prop might just do, however. A clever person with access to a 3D printer could possibly mirror a thrust propeller & print one out in polycarbonate.
The other thing about hydro-gens, of course, its that they only work when you are sailing. Good for driving the autopilot. But they won't keep your beer cold at anchor.