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 character was only mentioned in writing.
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.
As with many names on the "Imperfection" list, Sam Seaborn is an in-joke referring to the character from the TV series The West Wing.