A speaker walks down to a harbor in the early morning, just to breathe. The poem is short and spare, and it gives us a man who seems both attentive and empty. He keeps a journal of what he sees, so he is clearly a watcher, someone who records the small details of the port. Yet the way he describes himself is almost the opposite of busy or driven.
The line "content to rust" is the heart of it. Rust is slow, passive decay, and the speaker accepts it without complaint. He doesn't fight stillness or want to be improved. Then the ending turns the idea inward: "my mind is a hole / full of little things." A hole is empty, but here it is also full, so the mind becomes a place that collects odd scraps, much like the journal does.
The mood is calm, unhurried, and a little wry. The title, "Harboring," suggests both a port and the act of holding things inside, and the poem seems to rest on that double sense.