The dead man's really on his own out there in "Not Waving but Drowning." Because he's out of reach, physically or metaphorically, there's no one to throw him a line when things get tough. Even worse, being dead finally cuts off all hope of help or reconnection to the community of the living. Instead, he's stuck, alone, obsessed with his fate, while the fools he left back on land chat about his death.
By focusing too much on his own problems, the dead man isolated himself from society, so he has no one to blame but himself.
The ignorance and insensitivity of society are to blame for the dead man's isolation. If they had stepped up and paid attention, the tragedy of his death never would have gone down.