Communication
This is a poem about the failures of communication. Although the poem begins with a lighthearted tone, the final stanza confirms what the poem is really about. It is revealed that the old man has struggled for much of his life, presumably with his mental health. However, it is insinuated that his cries for help were never properly understood. Here, his drowning and the subsequent miscommunications are used as a metaphor for his inability to seek help for his struggles his whole life.
Isolation
The one impression we get about the drowning man is that he is incredibly isolated. His true feelings and struggles are misunderstood by those around him, and he continues to drown without help. Although this is described literally in the first stanza, this can be read as a metaphor for his mental health struggles and the fact that nobody helped him or truly understood what he was going through.
The boy who cried wolf
In the second stanza of the poem, we hear from those who knew the old man. They comment on the fact he would joke around all the time, which is partly why they thought the man was greeting them when he was really asking for help. This suggests that if someone jokes around a lot, they are less likely to be taken seriously when they really need help.