You can find the text of Sea Grapes at
http://nobelprize.org/nobel_prizes/literature/laureates/1992/walcott-poetry-seagrapes.html

Talking about imagery is relatively easy because you can discuss it in isolation from everything else. Discussing rhythm and form in a free verse poem like this is almost impossibly difficult because they’re so intimately bound up with the whole dynamic of thought and feeling that makes the poem what it is. And yet to ignore them is to ignore the things most essential to the poem’s life. For example, the impact of the opening line is determined as much by its buoyantly singing iambic rhythm as by the visual information it conveys. In the next line the reversal from iambs to trochees means that two stresses come together (“light / tired”). Two stresses coming together always slow things down. Here, the swift forward movement of the clear |l| at the start of “light” is suddenly weighed down by the resulting spondee (reinforced by the line ending) and the assonance of “light”, “tired” and “islands”. I’d say this change and its sheer suddenness (like a sudden collapse) does more than the word “tired” to express a sense of exhaustion. When an iambic rhythm is resumed in line three its character has changed, losing the buoyancy of line one. This is partly a matter of syntax – it’s a phrase in apposition to “that sail”, with a consequent stalling of the sentence’s momentum – and partly a matter of the sound of the words. And then in the next line, “for home” seems to me to come in like a trailing afterthought, isolated from the main push of the sentence by the line and stanza break, as if the poet imagines the boat or the men in it as so tired that it’s almost an act of will for them even to remember that that’s where they’re going. This feeling is, I think, only strengthened if you put a strong compensatory emphasis on the word “home” when you get to it. Of course I am assuming that the line and stanza breaks have meaning and should be registered in reading. If they don’t, why would the poet put them in? In a poem not written to a set metrical form there’s nothing to force him to.

I’ve said this poem isn’t written to a set metrical form, and I called it “free” earlier, but I’ve been talking about traditional metrical devices like iambs and trochees. Walcott uses traditional metres superbly. Freedom, in this poem, comes from his constant shifting of the metrical base. Some lines are clearly iambic throughout, some have a clearly trochaic base, others shift between metres in a more indeterminate way. In all of them, he is choosing his metre and choosing the length of the line for their expressive impact, not fulfilling a predetermined metrical scheme. The poem’s extraordinary rhythmic vitality and intimate fusion of rhythm with meaning depend on the way it combines the muscular strength of traditional metres with this freedom from any sense of being contracted to a particular metrical form.

They also both depend on and work in alliance with a brilliant syntactical orchestration that I’ll try to discuss later. This underlying syntactical strength is what makes the difference between a poetic representation of weakness, like the one at the beginning of line four, and a real failure of poetic impulsion.