As the previous response mentioned, this poem, and Larkin's style, relies heavily on metaphors. "The North Ship" is composed entirely of an extended metaphor about ships, and life. In Larkin's book, this poem is also titled "Legend," which leads us to believe that, as with other legends, there is a kind of message or lesson to learn from the story of the three ships. So, I want to discuss an additional summary of the poem in terms of what its metaphors could mean.
The poem's ships could represent life's many paths based on the different choices we make (each of the three ships "chooses" a different direction to sail).
The ship that sails west discovers "a rich country. We could see this ship as representing a life path that leads to wealth and prosperity--perhaps a path that leads to the more traditional ideas of what it means to be successful. The ship that sails east experiences some more tumultuous moments, ends up getting stuck somewhere, and is less successful. Both of these ships return eventually.
The third ship sails north, encounters an "unforgiving sea," and does not return, perhaps because it did not expect to return. Larkin tells us twice that this ship was "rigged for a long journey." Thus, this ship could symbolize the life path that leads us the furthest from our origins and our home, the life path that is perhaps the scariest and most dangerous and unfamiliar, as well as the most mysterious and full of potential.
Larkin's poem, "The North Ship," concerns the sailing of three ships. Each of the ships "goes sailing by" past the speaker of the poem. The poem depicts these ships as each undertaking qualitatively different journeys, experiencing success, hardship and suffering depending on the direction taken.
Starkly elemental and broadly (inscrutably) metaphorical, the poem portrays three different paths that may be taken and suggests that intentions and outcomes are intimately linked.
The style of the poem can be connected to imagism, a poetic trend of the early/mid 20th century that relied on specific images to convey meaning organically (i.e., without much exposition, if any).
In the poem, the speaker watches as the first ship heads west, "carried to a rich country." The second ship heads east "to anchor in captivity" and the third ship heads north.
The first two ships travel under a wind that is, respectively, helpful and violent. The third ship going north travels with no wind at all.
The third ship drove towards the north, Over the sea, the darkening sea, But no breath of wind came forth, And the decks shone frostily.
The northbound ship, unlike the others, is "rigged for a long journey." It does not return, as the others do "happily or unhappily" but instead journeys "far and wide/into an unforgiving sea.