This section is one of the most famous and important in the poem. It's also one of the most erotic.
A 28-year-old woman watches 28 men bathing naked in the river. She watches them from behind her window blinds.
She imagines that she is bathing with them in the river, as the "twenty-ninth bather."
Her gaze is erotically charged, like a hand passing over their bodies.
The young men don't realize that they are swimming around this "twenty-ninth bather." If they did, they would probably be embarrassed because she's a woman.
This section is intensely homoerotic. Many scholars believe that Whitman expressed his own feelings and desires through the gaze of the hidden woman. There are no comparable descriptions of naked women in the poem.