The speaker describes knowing other people almost too easily: their thoughts have become as visible as grey seaweed drifting in warm, shallow water. There is no mystery there, just clarity. But the person she addresses is different. She has never managed to understand them, and their hidden inner life feels like treasure—gold sunk long ago in a Spanish galleon, resting far down in cold, dark water. The contrast creates a quiet, wistful mood. Ordinary people feel open and predictable, while this one person remains distant and fascinating. The image of sunken gold suggests both great value and something lost or unreachable. Understanding them would be like recovering something precious from an unreachable past. The feeling is less frustration than the strange pull of someone who stays just beyond the reach of understanding.
Understanding — Summary
I understood the rest too well, And all their thoughts have come to be Clear as grey sea-weed in the swell Of a sunny shallow sea.
But you I never understood, Your spirit's secret hides like gold Sunk in a Spanish galleon Ages ago in waters cold.