I think, given the setting of this excellent story of survival, that we can safely assume he wasn't wearing a swimsuit! If you look at the text carefully, you can see that in the opening paragraphs of this story the dress of the man, who interestingly remains nameless, is described to us, in a paragraph that also makes clear the man's central failing in underestimating the power of nature:
Fifty degrees below zero stood for a bite of frost that hurt and that must be guarded against by the use of mittens, earflaps, warm moccasins, and thick socks. Fifty degrees below zero was to him just precisely fifty degrees below zero. That there should be anything more to it than that was a thought that never entered his head.
Thus, obviously, the man is dressed extremely warmly for the weather, but also note how this paragraph makes it clear the limitation of the man in treating the temperature and the intense cold very literally and precisely, being blind to the might of nature and the inherent weakness of mankind in response to the overwhelming power of the natural world.