The speaker is missing someone who comes and goes, and the poem is about the ache of waiting for them to return. She remembers being held and hearing "I want more time," and plays on that phrase: when they're together, time seems endless, but once the person leaves it speeds by. "Blink and you might miss something, / I blink and I miss you" is the neatest expression of that idea.
The central image is the sea and the shore. The loved one is like the tides, wandering and changing, while the speaker sees herself as the shore, staying put and hoping the water will come back. But by the end her confidence has slipped. Days become weeks, and she starts to wonder whether the comparison even holds, and whether the words she once believed were real. The mood is quiet, patient and lonely, with doubt creeping in at the close.