Through various poetic devices, "You Will Know When You Get There" conveys that each man must walk a final descent into death—and this is a walk which must be made alone.
Consider the primary metaphors: the sea and the time of day. The sun is disappearing, leaving only shreds of light. The sea is ominous, and at this point in the day, no one comes or goes from the sea. To reach it, the man must walk down a steep and slippery kilometer. Together, these metaphors convey that the man's life is ending, enforced by the repetition of the direction he is headed: down.
...nobody else goes down.
...and down you go...
Down you go alone...
Life is over. There is no further climb, challenge, or battle to be won. The last thing left for the man is to descend out of the land of the living and into death.
Also note the use of enjambment. This process is not a slow death, noted by the way sentences flow from one line and stanza into the next. There is no rhyme pattern and no meter. Sentences do not end at line breaks but in the middle of lines. This mirrors the "wet-metalled [steps] where a shower passed" that the man follows. The path is slippery and downhill, plunging him quickly to the end of the journey. Just as the man seems to speed down the sea, the reader speeds down the poem, reflecting the journey.
The youthful boys want nothing to do with death. The faces of young boys appear along the path, their youth juxtaposed with the man on his journey to show their opposite positions on life's journey. The boys are just beginning their lives with much left to experience; they are hesitant to acknowledge the man, who represents their own fates one day.
Finally, a door slams, a metaphor for death's final call. The man must finish his descent into the sea "alone, so late, into the surge-black fissure." His death is his alone to face. All light is gone and the metaphorical day of his life's existence is complete. The tone of the poem is solemn, conveying a resolute acceptance of the eventual fate of all mankind.