A speaker travels by night on a rusty ferry toward Kirkwall, and the poem follows the crossing as darkness closes in. The sea is the main threat: it becomes a "ferocious monster" that could swallow any trace of happiness and bury it under white foam. The image makes the water feel alive and hungry, and it shows how small and exposed the speaker feels.
Against that, the poem offers only small points of light. A dimly torched gate lets the speaker glimpse an unknown island, silent and dark, kept awake by a lonely lighthouse. The word "lonely" seems to mirror the speaker's own isolation.
By the end the mood is queasy and anxious. Sitting on a bench, feeling sick, the speaker repeats "Breath in, breath out" like a private instruction and nearly blacks out, yet stays present. The short, plain lines add to the sense of someone simply trying to hold on until the journey ends.