In just one line, this poem puts us on a ship in rough water, with someone too sick to stand. He lies flat on the deck, and the pitching motion tells us the boat is heaving up and down. Lying down is what a seasick person does to survive: he gives up on dignity and stays low, hoping the world will stop tilting.
The surprise is the last phrase, "among the humpbacks." Humpbacks are whales, so we might picture a whale-watching trip where he lies among real animals. But the word could also describe the rolling swells themselves, which rise and sink like huge backs, or the other sick passengers curled on the deck. The poem doesn't settle this, and that ambiguity is part of its charm.
The mood is a mix of misery and wonder. He feels wretched, yet the sea around him seems alive and enormous. Being so low and helpless makes him seem to belong to the whales' world, if only for a moment.