Sea Otter, otter native to the North American and East Asian coastal waters of the North Pacific Ocean. The sea otter is the largest member of the Mustelid (or weasel) family, which includes other types of otters as well as weasels, minks, badgers, ferrets, and skunks. The sea otter is the only type of otter that lives in the ocean. Scientists consider sea otters to be a keystone species, meaning they are important indicators of the environmental health of their marine habitats.