Jerruy - Length == easier target. You don't have to quote the whole thing, you just need to pick out the (quite obvious!) journey and then say "OMGZ look! It is imaginative" with some quotes.
Basically it is the inscrutable forces of nature sending the Mariner back to his own country?
The way I read that part was that the Mariner's learning to see beauty and worth in the water snakes (that were mentioned in part 4) has placated God and/or the spirit(s) of the earth, and part VI is the conclusion of the Mariner's journey home.
It shows the audience the mariner has a better appreciation of how to appreciate what is given to him by God/spirits (eg, the apparent enjoyment in "This seraph-band, each waved his hand : It was a heavenly sight !"), and the final rigors of the end of the journey ("Like one, that on a lonesome road Doth walk in fear and dread," stanza).
The description of the harbor bay is vivid and has connotations of goodness and cleanliness ("clear as glass", the calmness of "moonlight steeped in silentness") - on the journey's front, this shows the mariner has returned to where he has started, but it has changed in his estimation.
The use of heavenly hosts, walking dead, and the reference to the ship flying are all highly imaginative.
The actual "plot points" of part VI - mariner hears spirits discussing the amazing speed the ship is traveling (presumably it has actually been lifted from the waves: "Fly, brother, fly ! more high, more high !")
The rest of the trip is uneventful ("Swiftly, swiftly flew the ship, Yet she sailed softly too :") apart from the fact that the crew are zombies
.
Then the ship arrives home ("We drifted o'er the harbour-bar, And I with sobs did pray--")
A group of angels turn up to remind the mariner that he should be aspiring to greater things/to show the reader the awesome power of nature ("A man all light, a seraph-man, On every corse there stood.")
The angels leave when other humans arrive - the local pilot (look up a definition, like a harbor organizer, I think) and priest-equivalent hermit.
The End of Part VI