"Sea otter" is the common name for a specific species of marine mammal, Enhydra lutris. According to the rules of capitalization, proper nouns, which include the names of specific species, should be capitalized. This applies to both scientific names and common names.
Capitalizing "sea otter" ensures that it is properly identified as the name of a particular species and distinguishes it from other types of otters or aquatic creatures. It also shows respect for the scientific and common names used in the field of biology and zoology.