How does Queequeg’s coffin help Ishmael survive at the end of Moby-Dick?

In the novel Moby-Dick by Herman Melville, the coffin originally built for Queequeg when he fell ill serves as a vital survival tool at the end of the story.1

After the white whale destroys the Pequod, killing all other crew members, Ishmael is the sole survivor. He is able to stay afloat by clinging to the coffin, which functions as a makeshift lifeboat, until he is eventually rescued by another ship, the Rachel.1