Told in the present tense, the poem puts us inside the moment of drowning. The speaker describes the sea closing over their head, the current dragging them down, and their legs kicking in a struggle that doesn't work. The cold water seems to wrap around the body, and the speaker feels tiny, "naught but a grain of sand" against the endless sea. Gradually the kicking stops, and darkness takes over.
The feeling moves from panic to helpless surrender. Short, plain lines make the descent feel steady and unstoppable. The ending is the most striking part. We might expect the speaker's final thoughts to be of loved ones, but the poem says the opposite. The last thought is about "what shall become of me," a question about the self and what comes after death. That makes the mood bleak and lonely, and it leaves a sense of fear and uncertainty rather than comfort. The "midnight hour" suggests both the dark water and death itself.