It is both. Mammal is the class and Pinnipedia is a sub-order of aquatic mammals.
Pinnipedia.
Marine Mammals Order : Pinnipedia
Seals are in the order carnivora, their superfamily is the pinnipeds, and they specifically are pinnipedia otariidae
A type of seal, occurring in the northern and California species, of the family Pinnipedia.
Paws
A walrus belongs to the group of marine mammals known as pinnipeds, which also includes seals and sea lions. Pinnipeds are characterized by their sleek bodies, flipper-like limbs, and ability to thrive in both water and on land.
A. W. Mansfield has written: 'Seals of Arctic and eastern Canada' -- subject(s): Pinnipedia, Seals (Animals)
Marianne Riedman has written: 'Wild about otters' -- subject(s): Sea otter 'The Pinnipeds' -- subject(s): Pinnipedia
Sea lions are part of the Otariidae family, and the Otariinae sub-family. They are also classified as part of the Pinnipedia superfamily, and are often described as pinnipeds.
You are referring to earred seals or sea lions...which are the super family Pinnipedia family Otariidae. ...or sea otters family Mustelidae sub family Lutrinae
P. J. H. van Bree has written: 'Updated catalogue of the remains of Pinnipedia (Mammalia) in the collection of the Zoological Museum Amsterdam, University of Amsterdam'