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 assassination then nobody would have guessed. The meaning of "relevant" is a trivial issue. I suggest you don't knock yourself out trying to make any points about it having some direct relevance.

Here's Vincent Price reading it:
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PostScript, 21st November 2011
I'm not the only one to notice the resemblance to Moby Dick. Melville was the same age as Whitman and Walt had not only read his works, he had also written reviews. Coincidentally, he wasn't only born in the same year as Melville, he also died in the same year.

Here's a passage from Moby Dick.
"Oh, my Captain! my Captain! noble soul! grand old heart, after all! why should any one give chase to that hated fish! Away with me! let us fly these deadly waters!.. etc" Had Whitman already written this poem before Lincoln was assassinated, then made the most of the opportunity? I wouldn't be in the least surprised...
  / o-captain-my-captain