The cold thing is mostly centered around keeping above the line of hibernation, so they don’t sleep the season away. As Then says above, Selkie’s situation was from her core body temperature crashing suddenly more than the cold itself. In Sarnoth proper, the use of heating elements to stave off hibernation came about because in earlier days unscrupulous types who could stay up and about a bit longer than others would take advantage and break into homes or otherwise steal things. So they began looking into ways to prevent annual hibernation phases.

↓ Transcript
This is me trying to answer the "why is cold 'bad' when they live in freaking Lake Superior?" question.