Darkness (Symbol)
Darkness here symbolizes the overwhelming uncertainty of life. Small lights offer some relief from it, but the speaker expresses skepticism that those lights can offer much help, comparing them to misleading "fairy fires." Moreover, the speaker struggles to understand and interpret those lights, and has to assign them meaning based on her previous (and, the poem hints, inevitably flawed) knowledge about how the world works. Only the darkness—which is to say, uncertainty itself—is free from biases and errors in interpretation. It doesn't purport to carry answers or meaning, and in this way, the speaker suggests, uncertainty is the only thing we can trust.
Time (Motif)
While the speaker exists almost outside of time, simply observing the world as it moves around her, the same cannot be said of the people she observes. These people don't just react as time changes around them. Instead, they actually seem to create time as a social phenomenon, ensuring that everybody exists according to a cohesive timeline. The seamen, for instance, relieve one another from the watch, taking agreed-upon shifts. The "deep voice" who announces the changing hour is an even clearer example of the social construction and enforcement of time. Through these figures, Smith suggests that even something as seemingly empirically obvious and unchanging as time itself is actually constructed via a complex series of social negotiations and rituals.
