Might power the instrument lights if they are LED.I think this exists. I remember a towed device with a propeller attached to a generator to produce electricity. A variation that may be more practical might be to allow your prop to spin while underway (from the sails) and the prop shaft turns a generator.
It's not quite so simple as that. Tidal flow generators tend to return very little energy for the investment. And, as with anything in salt water, their maintenance costs are pretty high. That teal generation hasn't become more popular is due to these economic problems as much as anything else.There has been several types of tidal flow generators available for decades, most of which are very efficient. Unfortunately, in the U.S. the greenies has managed to keep them from being used, claiming they'll kill tiny micro-organisms that may become entrapped in the generators blades. Of course, these are the same people that use municipal sewage, which is discharged into our rivers and bays, causing many of the problems we now face with our dwindling water supplies. Ya gotta love it!
Not only that, he apparently would like us to give up using municipal sewage treatment and return to the glory days of dumping our chamber pots out the window into the street. I personally am more in favor of improving the sewage treatment infrastructure, rather than giving it up entirely.However, as you alluded, they also have the potential problem of killing both adult and juvenile fish. Situated at the mouth of a large estuary (SF Bay, Chesapeake Bay ,etc.) they could effectively wipe out the "nursery" function of that estuary, dramatically diminishing both estuarine and marine fish populations. But, if you want to sail on a sterile ocean, I suppose they're an option.
As has been mentioned he averaged 1.4kn over the course of the trip, with prevailing currents in his favor. While I am not a fan of electric power for the most part, a solar array and electric motor would probably give better performance.The Suntory Mermaid II - Wikipedia, the free encyclopedia is a wave-powered ship that has gone from Hawaii to Japan, so the energy is there to harness and the idea isn't as far-fetched as it might seem; but I would like a bit more information on how the OP wishes to do it.