Otters are part of the Mustelidae family, which is a family of carnivorous mammals that includes skunks, weasels, wolverines, and badgers.
Unlike most marine mammals, they don't have a layer of blubber. But, they do have the thickest fur of all animals. An otter pup's fur is too dense for it to swim underwater. So, their mother leaves them floating while she searches for food, until their adult fur grows in.
Baby otters are called pups. Newborn pups need lots of attention, and will stay close to their mum's until they've developed enough skills to go it alone - usually at 6 months.