Marine mammal such as the California — or Australian — (3,4)
I believe the answer is:
sea lion
(Other definitions for sea lion that I've seen before include "Is alone (anag.)" , "one often immersed in salty water" , "Aquatic mammal - code name for planned German invasion of England, 1940" , "Marine animal, used as a circus performer" , "Acrobatic aquatic mammal" .)