Hmm. I suppose that your critic presumes 'more than once' because of ' never '. Never seems a bit odd if considered only during the one instance-- after all, the swimmer, if he didn't drown, would eventually be rescued or reach land.
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MrPedanticI would agree that "never" here might suggest "not in any circumstances", i.e. "I am such a good swimmer, that in no circumstances will I drown (not even if you throw me into the sea, far from land)". It might also suggest "however long the period of immersion".So you don't really think the 'never' here is awkward?
MrPedantic
MrPedanticI was puzzled by the 'MORE THAN ONCE' part of that native speaker's comments. I thought what he/she wanted to say was that it was not really natural to use 'never' unless it happened at least once.
I'm not overfond of that "never"; but I suppose the author might want to convey the emphatic nature of the boasters' boasts.