Some truly marine mammals are aquatic predators

Apart from the truly marine mammals as mentioned above, which do not come out of water at all, there are other aquatic mammals with flippers, which are amphibious in nature to the extent that they return to land during breeding season and where they stay and breed. These aquatic mammals include:
Seals
Sea Lions
Fur Seals
Walruses

Walrus:

Walrus, which belongs to the order Pinnipedia, is also an aquatic predators. Since these mammals are underwater predators, they are often hunted for fishery protection, apart for their fur, skin, meat, oil and tusks in case of walruses.