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I was thinking that they must hibernate or something and thought that the photo must have been taken early winter.

I saw a PBS special over the weekend about winter in Yellowstone park.
The otters there stayed active all year. I believe otter fur is one of the warmest furs there is? They have 2 layers of fur, a coarse outer layer, and a fine dense inner one. That and a high metabolism rate keeps them active in the coldest weather. There always looking for the next meal.

All that playing serves a purpose other than just being part of their nature. Keeping moving and eating every chance they get helps them survive those frigid conditions.