Arnold uses extended metaphor in the first stanza of "To Marguerite--continued" to compare the sea to life in the line "sea of life."  The word 'enisl'd' is shortened, and Arnold uses it to create imagery of isolation, forming the idea that man's existence is like an island, floating in the "sea of life" alone.  In line two, Arnold reinforces both metaphor and imagery by conjuring a picture of "echoing straits;" the connotation of the adjective 'echoing' suggests emptiness, as if the land and space, from one person to another is so void and empty that it must echo.  A 'strait' is a geographical term that basically means a passage of water between two bodies of land, so in Arnold's poem, "the sea of life" and the imagery of the water separates humanity from each other. 

This stanza really reinforces the idea of isolation, especially in the final line of the stanza, "We mortal millions live alone."  Arnold even stresses the word 'alone' by italicizing it in the poem.  In short, this stanza from "To Marguerite--Continued" compares the sea to human life to reveal how disconnected and isolated people are from one another.