Skepticism is the act of doubting a state of mind. In the poem, Smith doubts the normal state of mind a suicidal man had. She gets into the characters head and reveals that he felt that he was "drowning" in the world. The man wants to tell everyone at the funeral how unhappy he is but he can't because he is dead and "lay moaning." Everyone on the outside thought that he was just happy as a lark. The metaphor the author uses is a man that has gone too far out in the ocean and appears to be waving happily to people on the beach, but in actuality he is waving hysterically because he is drowning. Smith is skeptical of all human beings and their fakeness. Smith knows that this world is a sad place, she is a modern writer.
Poets of the modernist era tented to use language that way quite brief but highly imaginative to express theme. "Not waving but drowning" is only twelve lines long, but because of the language we can derive the psychology of a depressed individual. Even the title is highly imaginative and it allows us to derive the poem's metaphor. The man is screaming to himself "I'm not waving, I'm drowning" which means he is