Anyone who has said "when my ship comes in" knows the phrase: it means waiting for good fortune to arrive. The speaker takes it literally and builds a small daydream around it. Once the ship docks, she imagines buying new shoes and running far away, which is a telling pairing of a simple treat with a wish to escape.
The middle stanzas turn darker. The ship arrives safely, but the speaker's plan is to unload its heavy cargo, lock something away in an "unbreakable chest," and throw away the key. The poem doesn't say what she wants to bury, though the imagery suggests pain or memory she'd rather never reopen.
The last stanza snaps back to the present. "For now I linger – hopeless" is blunt, and the ship is only "a dot on the horizon." The mood is wistful and weary, with hope that is thin but still there. The daydream is vivid, and the reality is waiting.