This sentence is a bit convoluted owing to the double negation.
How about this:
HE had only been in water of subzero temperature.
The question rightfully arises: What was the air-pressure there (freezing point varies according to pressure exerted on a liquid).
No, I think in your interpretation the sentence sounds different. It gives the impression that the poor guy didn't have a chance to try warmer water, while the original sentence somehow suggests that it was his decision not to swim in warm water.