Nobody heard him, the dead man,But still he lay moaning: (1-2)
These lines insist twice that he's speaking and twice that he's dead. It's a confusing state of affairs, to be sure. Why do you think the poem starts with these two contradictory states? Is it a haunted poem?
And not waving but drowning. (4 and 12)
The opposition here is between the lively activity of waving and the deadly one of drowning. Nobody knew (or knows now) that his life was in jeopardy. Life and death are so close to each other that one can be mistaken for the other. At least, they can be if your friends are too busy with their own games and struggles.
Poor chap, he always loved larkingAnd now he's dead (5-6)
Though the speaker here might not have been aware that the dead man had been dying for a long time, he or she (or they) is at least aware of the outcome. It's so strange, the speaker thinks, that someone who appeared to love life so much could suddenly be dead. This is a weighty subject, so no wonder the speaker has a hard time saying it in a way that sounds appropriate to the situation. Still, it's kinda funny. It just goes to show how little these folks really understood about the guy, huh?