You'd think that there couldn't be a clearer distinction than the one between life and death, but "Not Waving but Drowning" goes out of its way to muddy the water, so to speak. The focus, after all, is a talking corpse who just won't shut up even though the living can't hear him. Is he alive? Was he ever alive? The other people seem to think that he used to be lively, indeed, but the picture of the dead man that comes through by the end is of a nightmarish existence that was always like being dead. No wonder he's been moaning about it.
For the dead man, his entire life was one long act of dying.
The dead man's complaint is too melodramatic to gain sympathy; plus, if drowning is metaphorical, he might not even be dead. Just in dire need of some therapy.