Years ago, a mate of mine on his honeymoon in South Africa was challenged to an underwater swimming competition at the hotel pool by a male German who had 'befriended' him.
After being out-done by the German in the number of widths and lengths swum, my mate bet £500 (a lot of money in the 70s, although John was a millionaire) that he could swim under water for five minutes without coming up for air.
The German readily accepted the bet only to see John return to the pool after half an hour with mask, fins, cylinders, the lot.
The German wasn't happy, but paid up, and John didn't see him again for the rest of his honeymoon.