In “The Castaway,” William Cowper tells the story of a sailor who falls overboard in the midst of a storm, yet this story also illustrates deeper themes in the poem of depression and remorse.

The poem opens in the first-person, with the speaker beginning his tale of a man washed off the deck of a ship. Into the water he goes, bereft of hope and friends, never to return to his “floating home.” This doomed sailor loves his ship and his captain, but he will never see either again. He is an expert swimmer and courageous, but even these cannot save him. He friends try to help, but they cannot change the ship's course. The wind is too strong. They throw to him whatever they think might help, but nothing does. Finally, though, they are forced to turn away and save themselves from the storm. The doomed man continues to live for a while, and the speaker reflects on what it must have felt like for him to be deserted by his friends, knowing he is going to die, and finally feeling his strength give way. Eventually, he sinks beneath the waves, never to rise again.

This man, the speaker, continues, has never before been commemorated by a poet, but his name and fate are recorded in Anson's book. The speaker can relate to the man, for he, too, knows what it feels like to be drowning. He is not drowning in the ocean, though. He is falling “beneath a rougher sea” of depression and guilt. He is sinking into “deeper gulfs,” but he feels just as hopeless as the abandoned sailor. The speaker is perishing alone with no one to help him and nothing to calm the storm of his own emotions.

The theme of this poem, then, is the experience of hopelessness. In one case, there is no hope for a drowning man in a physical sense. No one can save him. In the other case, the speaker's despair is emotional but just as real and just as intense, and perhaps just as deadly.