Otters have an internal temperature of around 100 degrees Fahrenheit—nearly double that of the chilly waters off the northern California coast, where they can often be found. Unlike whales and other aquatic mammals, otters don't have blubber to keep them warm. Instead, they rely on two distinct characteristics of their fur for insulation: Its density and its texture.
Otters have the densest fur of any animal on earth—about 1000 times more dense than human hair. Each strand is sort of spiky so that it becomes entangled and matted. This thick, dense, matted layer of fur traps air bubbles that act as insulation close to the otter's skin.
When they dive down, the water pressure forces some of these air bubbles out, but meticulous grooming keeps the otter insulated. In fact, Heather Liwanag, a biologist who studied otter fur as part of her Ph.D. research at U.C. Santa Cruz, found that the fur works better than blubber at providing insulation in shallow waters.
Check out this video for a closer look at otter fur: