Of course, Masefield is doing nothing but describing the ships, their destinations, and their cargoes, but the great contrast here makes it obvious that he is implying a theme beyond mere descriptions. My understanding of his purpose is that he is saying that when we look back at the past and at exotic places, we tend to notice the unusual, the romantic, the glamorous. But when we look around us in our own time and place, we just see the very mundane and mostly uninteresting elements. Personally I would rather be a British union member on the steamship than a slave on that rowing ship or a scarcely more free sailor on the galleon. But it is human nature to dream of the long ago and faraway and think that those were much more exciting than our humdrum existence. Have you ever read Edwin Arlington Robinson's poem 'Miniver Cheevy'? This is exactly the attitude of the protagonist of that poem.