Nostromo's swashbuckling subtitle may carry a larger meaning, referring to other kinds of borders. Political borders feature centrally in the plot. And thematically, Conrad addresses the borders of the human soul, isolated from other souls like an island of subjective experience surrounded by dark waters (Conrad will use that image as his title for Part Two).
The sea also functions as a symbol in Nostromo, representing absolute
political freedom (both positively, as the ultimate "spirit of liberty"
and negatively, as the "ungovernable" mass
). The "seaboard" would thus represent the region where freedom encounters
and struggles with its opposite -- conquest and governance. A fitting subtitle
for a novel about man's struggle with political institutions and ideals.