In A Nutshell
Dogs may be thought of as either their own group, or a slightly removed counterpart to the cat. However, dogs (Caniformes) find much closer relatives in the water, not among cats (Feliformes). Sea lions and seals are not related to other marine mammals, but are actually Caniformes, just like bears and the domestic dog.
The Whole Bushel
Seals and the rather incorrectly named “sea lions” are carnivores, very similar to land predators. These animals are not in any way closely related to whales, but they do have a surprisingly close link to modern bears, dogs, and weasels. Cats compose an entirely different sub-order of the carnivore group known as Feliformes, which are not represented in water.
Pinnipeds possess massive canine teeth that may compete with the most powerful Rottweiler guard dog, and thus should be considered the oceangoing equivalent of a wolf or even a small bear. Their order name, “Caniformia,” literally means “dog-like.”
When you see one of these flipper-equipped aquatic animals, you should remember you are basically looking at a very specifically evolved, fish-eating water canine. When examined, the flippers of some pinnipeds will show remaining hind claws that rather resemble human nails. Maybe it’s time to start calling sea lions “sea bears” or “water dogs.”