don't make a life threatning BET.
They should have made a bet like that, the conditions they must be able to swim or bets made must have expertise in that field, they know that swimming is at risk of sinking.
I used to make bets that have challenges like the one in the OP story swimming 50 meters in the sea and back ashore again, we do 5 who quickly he gets The $ 100 prize is a fun challenge for us, but we have to make sure the expert participants in swimming, those who can't swim don't enter.
You may not understand what OP is saying, maybe since you're not from this side if the world, the guy that got drowned is a good swimmer, people from that party of the country are born swimmers, like they practice it from birth and it's their lifestyle.
The current of those rivers are excessive, they are best described as creeks. They do the swimming on a normal day, but it's very risky, the guy may not have had enough strength to swim against the current and maybe it got higher as he advanced to the middle, overpowering him and eventually drowning him as a result.