Mammal one might find at Argentina's Mundo Marino (4)

I believe the answer is:

seal

(Other definitions for seal that I've seen before include "Wax stamp" , "Fasten securely; aquatic mammal" , "Mammal seen swimming" , "Imprinted disk affixed to a document" , "Ratify" .)

I've seen this clue in The Washington Post.

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