A large predator of the sea that hunts Cattle Fish and any stray humans they can find. Cunning if not intelligent, they hunt in packs, have a keen sense of smell, and multiple rows of sharp teeth.
[but what are they? seals? dolphins? sharks? otters? wolves that can swim? squid with jaws? -- Desert viking]
[Pack hunting doesn't seem common in ocean ecosystem, though I'm not sure if this is because the evolutionary 'age' of ocean beasts being more primitive or if it's the terrain that doesn't favor pack tactics. Dolphin do fish in groups and etc, but that seems to be more because their prey fish travel in schools. In either case I'd go with a land animal that got morphed a bit and is either amphibious or returned to the water completely while retaining land animal habits. Maybe an attempt to create cowdogs that went wrong? Because without training the more herd related dogs turn feral pretty quick. Here, try this out -- Ginger]
There is another related species, waterdogs, that are close companions with humans. Their habits something like otters and seals. Fully mammal, so can come out on land, but are pretty awkward there and usually stayed close to water. Also pack animals and are used to drive Cattle Fish. They have furred skin webs between their hind legs creating a trusting tail. Forepaws also have some skin webbing back along their ribs. Have an odd hopping gait when on land due to the webs. Their depth of swim is limited due to lack of extended ability to hold their breath. Sea Wolves came from waterdogs from a series of self directed mutations from waterdog's frustrations, including thoses over the Cattle Fish evading control by diving below the waterdog's diving capacity. The mutations include more integrated swimming surfaces, ability to give live birth in water, and better lung and breathing structures so range and depth are much increased. The changes to their limbs make them much more awkward on land and the live birth changed their breading patterns so a land-bound breeding season ended. These effects with the extended range removed Sea Wolves from human contact to the point that humans couldn't contain them enough to form the human-pack ties of the domesticated waterdogs.
Cross breed animals of waterdogs and Sea Wolves are mostly sterile mules. Often very useful if can be controlled, but control is chancy. Very rare due to different breeding cycles and habits. Almost always born to wild packs where often is infant fatality, either to failure of early swimming or pack leader interventions. However, with human intervention, shallow water and captive Sea Wolf males, some have been born in waterdog packs.