<em>Repetition</em> is a literary device used to repeat the same words or phrases several times in order to emphasize an idea.
Every stanza in the poem begins with the same phrase. This phrase being "I must go down to the seas again", which is then followed by verses that start with the same conjunction (at least in the first stanza; each verse starts with "And").
This repetitive device emphasizes the speaker's desire to go to the sea.
