I would have read this between 2000-2005. It's a single scene from a book; it may even just be an epilogue setting up another book, or a "look for by this author" extract after the end of the novel.
There are no overtly fantastic elements that I recall, but I wasn't reading any fiction other than SF in that time frame.
The scene is a mole protecting a harbor; it's deep in a foggy night. (The fog may have been magical, or magically enhanced.) A couple of longboats (ships' boats) with padded oarlocks silently approach; soldiers slip into the sea and crawl up on the mole. It's possible that a patrolling sentry is quietly eliminated. (I believe the term "mole" is used in the text.)
One of the invaders is a leader, possibly the leader of the entire operation. From his viewpoint (he is the main protagonist in this scene) we learn that their goal is to release the chain guarding the entrance to the harbor so the bulk of the invading forces (still on board ships) can be unloaded directly into the city.