Metaphors and similes—some original, others traditional—help the poem vividly render the speaker's state of mind and surroundings.
Some of these metaphors are very subtle in their connotations and allusive echoes. For example, when the speaker describes the "love-cars" in the fifth stanza, he compares them to adjacent ships:
they lay together, hull to hull,
where the graveyard shelves on the town. . . .
This comparison feels appropriate to the poem's setting (a beach town), but it's also eerie and potentially symbolic. Is the speaker imagining these ship-like cars "hull to hull" in port, or sunken onto an undersea "shel[f]"? Either way, the shelf-like graveyard seems to mark a border between life and death in the same way the continental shelf marks the border between land and sea. Love and death, in this image, also seem closely linked—just as they are in the song the speaker is hearing ("Careless Love").
Meanwhile, the way that song figuratively "bleats" from a radio links it back to the heiress's "sheep" in the first stanza. (Bleating in the literal sense describes the noise made by sheep and goats.) In this way, the heartbreak the song expresses seems to encompass not only the speaker/listener but his surrounding community—even the local recluse and her animals. Later, the "moonstruck" eyes of the skunks seem to reinforce the poem's heartsick mood: moonstruck is a traditional metaphor that can mean love-crazed or simply crazed.
Perhaps most importantly, metaphors help communicate the depth of the speaker's suffering. The combination metaphor/simile in lines 32-34 highlights the violence of his emotions:
[...] I hear
my ill-spirit sob in each blood cell,
as if my hand were at its throat. . . .
In other words, he feels as sick and agonized as if he were strangling each of his blood cells, and each of those cells were "sob[bing]." He may also be implying that his depression brings thoughts of self-harm or suicide.
As if that weren't enough, he borrows a metaphor from Satan in John Milton's epic poem Paradise Lost: "I myself am hell." In other words, his own mind torments him as much as any divine punishment ever could. He also links his torment with that of Jesus in the Gospels: the "hill's skull" metaphor in line 26 alludes to the site of Jesus's crucifixion (a hill called "Golgotha," which is Hebrew for "skull").