An incident from 25 years ago comes to mind:

We were climbing Mt. Kilimanjaro. The trail came quite close to a stream at one point, there was a bit of a pond that two of the women in the group thought looked inviting. The rest of us expressed the notion that they were crazy but I don't recall any explicit discussion of the source of the stream.

They ran together and jumped in. They discovered that comfortable air a few degrees from the equator doesn't make a snowmelt stream (we were about 8000' vertically and 15 miles of trail from the snowpack) a reasonable temperature to swim.

To compound it we had seen other climbers a time or two so they only stripped down to bra & panties (in previous situations where there was basically no chance of others stumbling on our group they had skinny-dipped) and so they were not only wet but wearing bras soaked in water that we figured was in the 40's.