The smallest of marine mammals doesn't have a layer of blubber to keep it warm like so many other marine mammal species. Instead, it has the thickest coat of any mammal. Sea otters have between 850,000 to 1 million hairs per square inch. For comparison, a human has maybe 100,000 hairs on their entire head. The coat is necessary because sea otters live along the western coastline of North America in frigid waters. The hairs trap a layer of air that insulates the otter against the chill.
Staying warm is so important to a sea otter that they will hold their paws up above the surface of the water whenever they're at rest to keep them warm(ish).
Meanwhile, a mother sea otter such as the one shown here will often pull her pup up onto her to groom it, keeping its fur oiled and dry to help it stay as warm as possible. In fact, she will spend several hours a day grooming her pup and herself. A lot of hairs means a lot of time dedicated to keeping it just so!