The title, "Alienation," names the feeling, and the three short lines show it. The speaker describes being a stranger in a place where most people are strangers too. You might expect that to be a comfort, since nobody belongs, but the poem doesn't offer that. Being surrounded by other outsiders doesn't make anyone feel less apart.
The last line is the surprise: "body in a boat." The speaker shrinks to a single body, almost an object, floating on something that isn't solid ground. A boat suggests drifting, being between shores, belonging to neither land nor home. It's also a small, enclosed space, so the image feels both exposed and isolated.
The mood is quiet, cool, and a little lonely. There's no complaint or drama, just plain statement, and that restraint makes the feeling land harder. The poem leaves you with a sense of suspension, as though alienation were less a sharp pain than a steady, weightless drifting.