Completely illogically, I wear mine on tidal rivers, but not nontidal (which are frequently deeper).
This summer, I travelled for a few days with another singlehander, even older than me, up the GU, through Leicester and down the Soar. She was wearing a lifejacket. At one of the locks, the toggle became caught in the paddle mechanism, and the thing inflated - I'd never seen this before, and it was quite alarming, if hilarious. She had a spare, but didn't wear it, as she was now travelling with someone. After a couple of days, I said I hoped she wasn't relying on me as a lifejacket, as I can't swim a stroke. "NOW YOU TELL ME!!!"
Iknow, I know........
Mac