This one isn't really set at sea. But there's a boat on the cover! Does that count?
Seriously though, the book was the first in the Deverry series to open the world up to regions beyond a few hundred land-locked miles, so it counts since the plot involves a lot of sailing back and forth to first capture and then rescue Rhodry.
It's the best I can do. The only books I've ever read that are in fact "seafaring" are Moby Dick and the Confessions of Charlotte Doyle, neither of which are anywhere near a favorite.