But still he lay moaning: (2)
"Moaning" gives us the first evidence that the man, although dead and "still", continues to feel pain. Whether it is physical or mental pain isn't clear, especially because it's hard to figure out how he could still be talking or feeling if he's dead. Magic? Weird science?
And not waving but drowning (4 and 12)
The poem's refrain insists on a terrible and fatal kind of suffering. It's bad enough to feel that sensation once, but to feel it all your life? Horrifying. What might drowning represent metaphorically?
It must have been too cold for him his heart gave way, (7)
It's not that these people don't believe the dead man suffered, it's just that they think it was only at the moment of death. They explain away his situation by coming up with physical reasons for his pain and death. The cold to them is a literal cold, as in water that's too frigid to swim in. The failed heart is also only a physical failure, not a metaphorical heartbreak. Their problem, see, is they didn't read enough Shmoop guides to poetry.