The main theme in Derek Walcott's "Ebb" is the conflict between man and nature, and this conflict is made evident in—and developed through—several key images. In stanza two, Walcott describes a "shoreline littered / with rainbow muck, the afterbirth / of industry." These lines evoke the image of a devastating oil spill. The "rainbow" is usually symbolic of hope and of peace, but here Walcott uses it contrarily to suggest the technicolor glimmer of a reflective surface of spilled oil. The conflict between man and nature is also made evident in the image, in stanza four, of "an oasis / marked for the yellow Caterpillar tractor." The tractor here is a symbol of man, and the oasis is a symbol of nature. The latter is "marked" for destruction by the former.
Throughout the poem, Walcott also uses images which connote the damage man has caused to nature. For example, in stanza six there is "a lamed heron" and "an oil-crippled gull," and in stanza eight there is a "washed-up moon... her radiance thinned." These images suggest that nature has been crippled and drained by the man-made process of industrialization. In stanza nine, Walcott describes the "palm fronds signal(ling) wildly in the wind." Here the palm fronds are personified to suggest that nature is frantic and alarmed at the prospect of its continued demise.
A related theme in the poem is the hopeless inevitability of manmade destruction. Walcott suggests that there is little or no hope of mankind stopping its exploitation of the natural world. He says, in stanza nine, that we are "bound elsewhere," the "elsewhere" here perhaps indicating mankind's obsession with industrial progress. The implication is that mankind prioritizes progress over stewardship of the natural world. Also in stanza nine, Walcott writes that "The schooner's out too far." The schooner here perhaps represents human society, which is too set in its ways—"too far" advanced along its path of industrial progress—to pull back now.