Oceans, boats, sailing—travel is everywhere in this poem. The speaker tells us that, before he met Helen, he felt like a guy roaming the ocean unable to get home (lost, confused, tortured). Her beauty, however, put an end to this long, hopeless part of his life and allowed him to finally get home. That, at least, is what he suggests. In addition, Helen's beauty transports the speaker to travel in another way as well. It reminds him of the "grandeur" of ancient Rome and ancient Greece. In a way, it allows him to travel back in time (metaphorically, of course).
Lines 2-5: The speaker compares Helen's beauty to "those Nicean barks of yore," which "bore" a "weary, way-worn wanderer" home. Since he uses the word "like," this is a simile. The repetition of the words that all start with the same sound (W: "weary, way-worn wanderer") is an example of alliteration.
Line 6: The speaker says he roamed the "desperate seas" for a long time. We also don't think the speaker literally roamed the seas so this is a metaphor for the difficulty and hopelessness of his life before he met Helen. It's a version of the whole life-is-a-journey thing. While "desperate" means "hopeless," we can't help thinking the speaker is personifying the seas a little bit, by which we mean he's giving a non-human thing (seas) human qualities (emotions). The speaker is the one who feels desperate, and he transfers his emotions to the ocean.
Lines 7-10: Helen's face, hair, and "Naiad airs" bring the speaker back home to… Greece and Rome. Weird, we know. This whole time travel thing is a metaphor for the way in which Helen's beauty reminds the speaker of two famous, ancient civilizations. Helen's beauty doesn't literally carry the speaker, so this is also a metaphor for the way in which beauty moves us. The repetitions of the H-words ("hyacinth hair") and the G-words ("glory," "grandeur," "Greece") are examples of alliteration. Finally, the rhyme on "face" and "Greece" is a slant rhyme (or half rhyme). It is also sometimes called an imperfect rhyme.
Lines 14-15: The speaker says Psyche is from "regions which / Are Holy Land." It sounds like she's travelled from some heavenly realm to come visit earth for a while.