S. Seaborn was a Starfleet officer assigned to USS Voyager, a lieutenant who was killed in the line of duty sometime before 2377. Seaborn's name was seen on a casualty report of all who died during that starship's journey through the Delta Quadrant. (VOY: "Imperfection")
This name was devised for the list; it was not mentioned as a casualty in any other episode. It is possible this name refers to one of the officers killed en route when Voyager was dragged into the Delta Quadrant.