The poem is about the decay of its own structure.
Each of the first three lines of a stanza has the same number of syllables, but the number decreases: 6, 5, 4, 3, 2. Th fourth, repeating line "of the old [monosyllable] ship" begins to break down as the poem continues, eventually petering out to one word blocks. In the first four stanzas, the last word of the third line must be only one letter different than the previous word of the same position, but this too decays, disappearing by final stanza. The poem itself narrows, drips down the page (hence the title).