In stanza one the captain and his sailors arrive at their destination point when suddenly the captain falls cold and dead, just short of his final destination. If I were to judge why he ended up like that I would say the captain stole a treasure, which belonged to someone else.
In stanza two the speaker talks about how the captains followers mourn his oh so "tragic" death. The third and final stanza states
"My captain does not answer, his lips are pale and still;"
Ok! so I have a confession, I started out trying to paraphrase this using no symbolism at all but I couldn't. If you haven't noticed by now I have set up my paraphrase using slight clues of symbolism. Now I will explain what everything means.
The treasure signifies the thoughts, words, or ideas in which someone could steal from you. The death of the captain is that person's fate if they do steal your words, thoughts, or ideas. No not death, but the sure wont be proud of themselves. The line that I quoted in stanza three means that if that person steals your words, thoughts, or ideas than he or she will not be able to speak or think for themselves.
This poem talks about the death of a captain who stole a treasure and is cursed with death. That is the price he pays for stealing something that belongs to someone else. I believe Keating wanted his students to get out of that poem just as I did.