Clearly the answer to this question lies in the way that what begins to be a beautiful sight of boats starting a race suddenly becomes something much more meaningful and grim thanks to the imagery employed towards the end of the poem. Consider the way in which the sea that the boats move through is compared to bodies strewn around that are mercilessly cut through as the boats advance:
Arms with hands grasping seek to clutch at the prows.
Bodies thrown recklessly in the way are cut aside.
It is a sea of faces about them in agony, in despair...
The race that the speaker observes is thus shown to be a symbol of the human struggle that we are all part of, and in which thousands are trampled on and cut down as others advance ahead of them. Let us remember that this poem was actually written during the Great Depression in America, and thus we can see how this central image corresponds to the call for social justice and equality that is inherent in the poem and its tortured description of the have nots who are crushed by the haves in society.