The speaker lists the blankets on their bed, each one a shade of blue taken from something in the world: a clear winter sky, a river, the Atlantic, and finally the eyes of someone they love. It sounds like a cozy picture, but the speaker is too warm to sleep and keeps tossing and turning.
The poem then tries out comparisons for that restlessness. It isn't like a plane in the sky, a salmon fighting upstream, or a buoy in a storm. Those images are all grand and purposeful, and the speaker rejects them. What they choose instead is small and a bit sad: a piece of grit in a drop of salt water, cut off from the blue of your eyes.
The feeling is longing. The blankets surround the speaker with blue, yet the one blue that matters is missing, so the comfort doesn't work. The mood is quiet and tender, with a gentle ache. The plain, song-like wording makes the loneliness feel honest rather than dramatic.