The speaker is awake in the dark, and the poem starts with a small, almost polite admission: sleep is wonderful, but it has limits. It can restore a tired body, yet it can't touch "the sea." Here the sea seems to stand for something huge and restless in the speaker's mind, whatever it is that keeps them up. Rest can't settle it.
The next lines complicate things. The sea is "no ship" the speaker might "obtain something from," so it offers nothing to climb aboard or take away, no easy answer or cargo. It is simply there. The closing words, "empty / and with much more than I know," hold two feelings at once. The sea looks empty, but it holds far more than the speaker could understand. The mood is quiet, humbled, and a little uneasy. Insomnia here feels less like panic and more like staring at something vast and admitting how small your own knowledge is.