According to Walt Whitman, this is about the assassination of President Abraham Lincoln. It is hard to see anything particularly relevant to that occasion but the American people accepted this poem as a suitable tribute and it became very popular. I suspect it owes a lot to Moby Dick - Melville was the same age as Whitman.
Afternote - "relevant" means having some real connection with the matter being discussed. There may be a metaphorical connection but if Walt hadn't said that this was about Lincoln's
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