Fun fact: this poem was based on Stevie Smith’s childhood memory of a newspaper article.
Ironically, she seems to have remembered it wrongly. Her biographers could only locate one similar story. There was a swimmer who was “rescued”, but they were actually waving not drowning.
https://www.google.com.au/search?sca_esv=557329753&hl=en&q=inauthor:%22William+McBrien%22&tbm=bks