"Leaving the Island" is a villanelle, so the first line of the first stanza -- "we roll up the rugs and strip the beds by rote" -- is also the last line of the second stanza. The last line of the first stanza -- "the ferry is no simple pleasure boat" -- is the last line of the third stanza. These two lines are alternated as the last lines of stanzas four and five. The final quatrain uses these two lines as the final rhymed couplet: "We roll up the rugs and strip the bed by rote. The ferry is no simple pleasure boat."
The first stanza establishes the theme of death as "summer expires," and this loss leads to tidying and cleaning -- rolling up rugs and stripping sheets as one might do after the death of a loved one in a once lively place. Now it is one of sweeping up memories, and "the ferry is no simple pleasure boat," perhaps calling to mind a hearse, a vehicle of utility and loss, leading them away. The second tercet continues to enhance the hearse-like quality of the boat, but she writes, "nor are we simply cargo, but we'll float," indicating dead wood that floats and this is followed with a repetition of "we roll up the rugs..."
The third and fourth stanzas move from the island to the landscape beyond, accompanied by the change of clothing and season. She speaks of seeing beyond the island to "land whose lines glaciers wrote," and this summons the image of ice and the distant past that is no longer alive but just the "muse of memory." The playful swimsuit will be shed for a "woolen coat," indicating that frivolity is being replaced by mundane and utilitarian concerns because the "torch of autumn holds but small allure."
By the fifth stanza, the reality of loss in the poem is well established and is reflected upon in the line: "The absences these empty shells denote/ suggest the losses winter has in store." The reader then understands that with this passing, "the songs of summer dwindle to one note," in the final quatrain. That note is sounded by the fog horn, which summons up the idea of finality. This is a last word that precedes the repetition of the couplet, reminding the reader it is time to roll up the rugs and get on the ferry, perhaps across the river Styx, and this "is no simple pleasure boat."