A seal arrives at an oyster bed, probably to feed, and then does something unexpected: it stays. The poem is only five short lines, but it follows a small change in purpose. The seal comes with a clear reason, since the bed is "good" and "broad," a plentiful place to eat. Then the hunger seems to fade into the background, and the animal is left "to stir sand," which sounds idle and aimless, even a little playful.
The title, "Beach bum," makes the joke explicit. We think of a person who hangs around the shore with no plans, and the poem gives that label to a seal. The speaker seems amused and affectionate rather than critical, watching the animal with quiet delight. Nothing dramatic happens, and that is the point. The mood is relaxed and gently humorous, like watching someone who has no reason to hurry. The short lines and the soft, loose ending mimic that easy drifting, leaving the reader with a small, pleasant picture of contentment.