ships. Subject of are descried.
3. towers of sail. Filled with the breeze, which has sprung up in the night. This line forms a predicate completion of are descried (line 4).
6. darkling. Dark, a poetical form.
7. but. Except, that—not. Did not dream but that, each, etc.
9. E’en so. Just in the same way differences in opinion arise between friends.
13. Unknown to each other, both came under different influences that changed the course of their opinions and beliefs.
16. dawn. When their beliefs were made known.
17. To veer, how vain. When people have drifted apart in their points of view it is vain for them to try to change their course to come together again.
19. one compass. Conscience.
22. parting. Nominative absolute.