In this moment, there was "a sad lingering music", which was probably the swans singing (it is said that swans sing when they are dying). And he was, the beaten swan was now dead, and his body "bobbed on the river like children's little boats". I believe that with this phrase, Dyment tried to express that this was, is and will always be a common and natural process in life and nature. And what is extraordinary about it is the fact that it happens everyday (like children's boats which are at all times on the river) and no one pays attention to it, even though there might be pain, violence, aggression and sadness.
Nevertheless, I think the poem is metaphorical; it is also comparing and showing the antagonism between victories, triumph and success with defeat, shame and humiliation. The poem is clearly divided in two parts, were these sensations are clearly felt and recognized. …