an animal that lives in water, and uses its fins and tail to swim.——朗文当代大辞典

In popular language, any animal living exclusively in the water; primarily denoting vertebrate animals provided with fins and destitute of limbs; but extended to include various cetaceans, crustaceans, molluscs, etc. In modern scientific language (to which popular usage now tends to approximate) restricted to a class of vertebrate animals, provided with gills throughout life, and cold-blooded; the limbs, if present, are modified into fins, and supplemented by unpaired median fins.——牛津当代大辞典  @en