There's a surprising number of voices in this short poem, but an even more surprising lack of real dialogue. No one seems to be connecting with other people in "Not Waving but Drowning." Instead, the dead man speaks to an audience that doesn't hear and vainly tries to correct misinformation, while the ones he addresses say untrue things about the dead man and talk over each other. Everyone seems to be stuck inside their own perspective, which makes this poem an awfully bleak picture of society in a microcosm of just twelve lines.
The dead man is to blame for the miscommunications because he pretended to be all right instead of letting people see how he really felt.
Nobody in the poem can communicate effectively because they're not interested in each other's perspectives. Everyone's doomed from the get-go.