A navy captain ends up in the desert, far from any sea, and the poem quietly asks us to notice how strange that is. He has brought seashells with him, a small piece of the ocean to keep him company. The speaker says he "deserves a little ring of life," which sounds tender, as if the shells make a modest circle of comfort around him in all that emptiness.
But the last lines turn dry and a little ironic. It is "convenient" that a road leads somewhere else and leaves him here, stranded. The word "convenient" seems to hide the opposite: the road is how everyone else escapes, and he stays behind. The title, "Beachhead," suggests a place where a landing begins, yet nothing here begins or arrives.
The mood is spare, wistful, and faintly sad. The speaker seems sympathetic but also detached, watching a man hold onto the sea after it has gone. Judging by the imagery, the poem is about displacement and the small things we use to feel less alone.