É uma metáfora do afogamento no sofrimento:
The hero of the story, adapted from the novel by Christopher Isherwood, explains himself in voice-overs: “Just get through the goddamned day: bit melodramatic, perhaps, but then again, my heart has been broken. Feel as if I’m drowning, sinking, can’t breathe,” he says.
( source )
Ele se afoga sozinho e em silêncio:
The theme of silence which resonates subtly throughout A Single Man, of course, is also a reminder of the real tragedy of George’s loss: that his grief has been censored by a society which demands his invisibility. He is not allowed the benefit of condolence and ritual by participating in Jim’s funeral; he is excluded and denied the status of an official “family member.” His last sixteen years with his partner have been omitted from the record. Through this erasure by uncontrollable external forces, as demonstrated by the repeated metaphor of a man quietly drowning in the ocean, the film suggests, especially in George’s final moments, that suffering is most extreme when experienced in silence.
( fonte )
Também leva a ironia quando ele está realmente se afogando:
The swimming motive is actually what starts the movie. Drowning to be exact. As if George was drowning without love, without Jim, without something that made his life right, important, valuable. He daydreams about it during his lecture as if he wanted to drown completely, disappear. It is ironic, and it is not the first irony in the film, that after he will actually be drowning during the swim with Kenny, he will realize how valuable life itself, moments we have are.
( fonte )