In "A Song of Despair," the woman, who the speaker used to love, is compared to the treacherous sea, which swallows up pirates, sailors and sunglasses alike. The man who was foolish enough to fall in love with her is kind of like those sailors who used to sail over the horizon before they knew the world was round. In other words: a big, old silly-britches.
The poem sets up woman as the sea and man as the seafarer, and his foolish attempt to navigate her is his folly. Way to go, man.
The poem makes the argument that all romantic relationships are doomed, and are therefore all follies. Yeah, good luck with that.