Alliteration fills the poem with bright, happy music that evokes the speaker's joy. It also brings scenes from the speaker's evening to vivid life on the page.
For example, the bold /b/ sounds of "bare and bright" call readers' attention to the ferry's austereness; this isn't a lush, cozy boat but rather something utilitarian and starkly lit. The ship "smelled like a stable," the speaker continues, the hissing sibilance perhaps conveying the speaker's disgust.
The alliteration in lines 4 and 5 is gentler. Listen to the delicate /l/ sounds here:
But we looked into a fire, we leaned across a table,
We lay on a hill-top underneath the moon;
These soft sounds suggest the sweet beauty of the evening, something that persists despite the speaker's stark surroundings. Alliteration also makes the lines more memorable, helping the poem linger in the reader's ear just as the scene lingers in the speaker's mind. The parallelism and anaphora of the lines add to the effect: the speaker says "we did this, we did that," creating a soothing, predictable rhythm.
The alliteration of lines 11-12 again enhances the poem's imagery:
And the sky went wan, and the wind came cold,
Those quiet /w/ sounds might suggest a quiet reverence on the speaker's part as the inky blackness of night gives way to the gray of the dawn, while the crisp sounds of "came cold" evokes the sharp chill of the wind.