The sea has often been used as a metaphor for isolation in literature. The vastness of the sea is juxtaposed with the smallness of man. While man can see for miles across the sea, he is, in reality, far from the next individual in both literal and figurative terms. The speaker in the poem "To Marguerite" is realizing just this.
First, the poem present itself as a personal reflection directed at a possible friend or lover from whom the speaker feels removed somehow, "enisled" in the "sea of life" (ln. 1). This sea is constantly moving, sometimes bringing beauty, the "lovely notes" (ln. 11)from the nightingales, but then the sea just as quickly sweeps the speaker out of reach. This image suggest that the speaker, and mankind, is powerless to control his movements and that the sea, a hand of fate, is purposely keeping the him from reaching his soulmate.
The hope provided by the idea that everyone must have once been "Parts of a single continent" turns to longing and even despair for the speaker as he ultimately realizes in the last line that he is alone in the "unplumb'd, salt, estranging sea" (ln. 24).