Lyric discussion by dezdenpetty 

Cover art for Swimming Song lyrics by Loudon Wainwright III

I think it's about being an alcoholic, pretty much definitely. First off, he was well known for struggling with alcoholism. And secondly, the metaphor of swimming is so often used to represent alcoholism in songwriting that it's pretty obvious. Here are some points to back it up:

'Salt my wounds, chlorine my eyes, I'm a self-destructive fool'...this shows that he knows it's bad for him, but he still does it anyway

'This summer I swam in a public place And a reservoir, to boot, At the latter I was informal, At the former I wore my suit, I wore my swimming suit.'...this says to me that when he's in public he's being polite and formal by 'wearing his swimming suit', aka keeping his drinking under control, but when he's on his own at the reservoir he's 'informal', which I'm assuming means skinny-dipping, aka being reckless and wild with his drinking.

This is again emphasised in the final verse: 'This summer I did swan dives And jackknifes for you all And once when you weren't looking I did a cannonball, I did a cannonball.'..showing that when in public, he does more elegant and sophisticated moves like swan dives and jackknifes, representing him faking keeping his drinking under control, but then 'when you weren't looking' he does a cannonball, suggesting he loses control of his drinking when nobody is around.

@dezdenpetty Sorry it took me 6 years or so to reply. I've heard many versions of this before I searched here to see what other people thought the lyrics meant. I was thinking it was about life in general and its many challenges. We are all swimming in it after all. I am sure the artists who have covered it may also have that view. That the whole thing is about alcoholism is perhaps wide of the mark. I am sure it is about growing old and the learning curve. Each verse an aging process. Perhaps the last one is...