Not Waving but Drowning
How we cite our quotes:
Nobody heard him, the dead man,
But still he lay moaning: (1-2)
The very first thing we learn in the poem is that the dead guy has no audience. So why's he talking? Furthermore, how is he talking if he's, you know, dead? Depending on how you see things, he could be in some kind of afterlife or a ghost, desperately trying to tell his story to the living. "Still" suggests a continued effort at communicating, even if it's in vain. Spooky, but sad. It might be time to call in that lady from Poltergeist.
I was much further out than you thought (3)
The dead man addresses a "you" here, confirming that he has a target audience in mind. And it's more than a little accusatory. He would tell them what really happened, if only he could get through. Alas, it's probably too late. He's dead, remember?
And not waving but drowning. (4 and 12)
The biggest miscommunication in the poem is the impression that the dead man was waving, when really he was probably, you know, flailing. Why would onlookers see a playful greeting instead of a sign of distress? Especially if we take the drowning as a metaphor, how could they miss that he was struggling for his entire life?