When I was doing road trip through Iceland in may cca 10 years ago, I ended up in northern part in some sort of camping where there was just my tent. Plus surprisingly some local kids were on some school trip.

I dont even recall how the heck I ended up with them in their trip bus, going to some canyons. Everybody got wet suit (around 5mm), and we ended up jumping from cca 5m cliff down to river. There were still patches of snow on the banks and water temperature was corresponding, most inland of Iceland had still metres of snow, only ring road was cca passable.

Most boys and me kept jumping and climbing back up, it was almost same cold in wet wet suit next to river than swimming in it. Most girls shivered like crazy, blue lips and all.

These were icelanders, around 13 years old, tough as it gets re cold. I recall their teacher explained it to me that in the past, as test of maturity every boy (not sure about girls) had to swim across big fjord somewhere around Husavik IIRC as sort of rite of passage, at least 3km width.

Glad nobody had heart stopped, that river was murky and strong. Every jump into that cold was like an electric ahock to mu body