I solved the century-old mystery of a shipwreck survivor

A good tall tale has an element of plausibility. A 6km swim is a common workout for a college swimmer these days. If the river conditions were favorable, the story on its own was not suspect.

I mean, there is this Icelandic guy

https://en.wikipedia.org/wiki/Gu%C3%B0laugur_Fri%C3%B0%C3%BE...

who actually did swim 6km in freezing waters after his vessel sank in 1984..

Yeah, but that guy was well insulated with blubber.

A 6km swim in a 78F pool is a common workout.

I've repeatedly swum in Lake Michigan (looking online, it has comparable temps to the St Lawrence in Quebec) as early as Memorial Day. The water temp is often in the mid 50s, very cold even with a full wetsuit, hood, and prep, but feasible as a ritual with which to open the season and start the summer. This year, in May, it was 45F/7C. Insanely, painfully, shockingly, unsafely cold...I decided to break tradition.

In July, a 6km swim could be fun! In May, depending on the climate that year and the swimmer's metabolism and subcutaneous fat levels, it might be survivable. You might lose a few extremities to frostbite. Or it might not be survivable. 1 in 800-something odds, with an athletic 29-year-old being the only survivor, seems reasonable enough.

It is a good tall tale, out on the murky limits of credibility.

It's pretty impressive (and not always widely known) how much cold water impairs swimming ability. There are some good videos on this site - coldwatersafety.org - of Coast Guard volunteers trying to swim in 45deg water. They lose motor function before making it 20m or so. Only one is able to make the short distance to shore, and he is one of the largest guys and a professional rescue swimmer. A 6km swim in 45F water for someone that is not wearing protective clothing or well acclimated to cold water is not realistic.