Matthew Arnold's "To Marguerite--Continued" is a romantic poem (both literally and figuratively), filled with passionate despair regarding the sea's isolation, a metaphor for his possible relationship with the woman to whom it is dedicated.
The poem title looks like a letter, or a post-script of a former, longer letter. Indeed, another poem "Isolation: To Marguerite" precedes it:
[Arnold] travelled to Switzerland in the course of his duties. There, in 1848-49, he met the "Marguerite" who inspired "Isolation: To Marguerite" and "To Marguerite--Continued." No one knows Marguerite's real identity, although there is of course a great deal of speculation; some critics even insist that Marguerite is an imaginary figure.
Later, in 1851, Arnold married someone else, so he did lose Marguerite, if she ever existed.
The poem is heavy in personification, imagery, and metaphor, as it likens humans to the seas:
Oh! then a longing like despair
Is to their farthest caverns sent;
For surely once, they feel, we were
Parts of a single continent!
Arnold says that modern man is like the current watery world, isolated and disconnected.