I have included the sentences I was most doubtful about.

Question

1) The ship was cheered and they left the harbour. They went below the prow (I think "kirk" is the archaic word for "prow")
2)The mariners were then caught (can you say "struck") in (not by) a violent storm, then by mist and snow and finally were surrounded by ice.
3) The storm is personified as a huge bird, chasing the ship with its large wings. The ship drove fast southwards with its sloping masts (I need a synonym for "sloping" and "dipping") and dipping prow chased by the loud storm.
4)The ice growled like a dog, roared like a lion and howled like a wolf.
Colerdige uses metaphors taken from the animal world to express the sounds made by thr ice.

Answer 1

1. "kirk" usually means "church," and I think that's what it means in this context.
http://rapgenius.com/Samuel-taylor-coleridge-rime-of-the-ancient-mariner-lyrics#note-80646
Use Ctrl + F and type in KIRK -- and then hold your cursor over the word to see it defined. You can also find definitions here: http://en.wikipedia.org/wiki/Kirk

Then look at that whole stanza --

The ship was cheered, the harbour cleared
Merrily did we drop
Below the kirk, below the hill
Below the lighthouse top

To me, the image is that as the ship left the harbor, they seemed to go downhill in relation to where the church (cathedral?) and the top of the lighthouse was.

Now ... all that said ... how will you rephrase the first part of that first line?

Answer 2

2)The mariners were then struck by a violent storm, then by mist and snow and finally were surrounded by ice.

3) The storm is personified as a huge bird, chasing the ship with its large wings. The ship sailed fast southwards with its billowing sails and with its prow going violently up and down, chased by the loud storm.

4)The ice growled like a dog, roared like a lion and howled like a wolf.
Colerdige uses metaphors taken from the animal world to express the sounds made by the ice.

Be sure to read very carefully to catch all the changes I made.