tongue firmly in cheek??
I stuck with this expression:

He wrote, tongue firmly in cheek: "It's not dangerous to swim here"

Some idea of what that means?

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Very grateful for your help

Thank you DavidK... I was wondering whether that expression needed some correction

I am stuck on this expression. (corrected) This expression puzzles me. (better)

Thak you Matthew!   I see  that you know chilean spanish haha!

The expression "tongue in cheek" is used to denote irony or sarcasm.  In this case, we can assume that it is in fact dangerous to swim there.  ¿Cachai?