A few thoughts...
Odobenocetops was a genus of Miocene Odontocete (toothed whale), related to dolphins, which was bulky in build, had tusks, and may well have been aggressive. They were carnivores (but certainly had their predators - they lived alongside such icons as Megalodon and Livyatan), but a couple million years of evolutionary modification could easily see them take on a more omnivorous diet, should one food supply drop.
Walruses have tusks, are bulky, and are aggressive. They are not, however, fully aquatic, or omnivores - however, both of those things can be changed. Walruses appeared in the middle Miocene epoch, about 15 million years ago, so that's about enough time for a transition from semi-aquatic to fully aquatic. Perhaps, individuals with better adaptations for swimming were selected for until this change occurred, and then the newfound greater range at sea allowed populations to migrate to warmer waters where there's at least some aquatic flora (seagrass plains, I'm thinking), and thus a transition to omnivory occurred.
Lastly, a third option.
A population of pigs in the Bahamas, specifically on Pig Island, are world-famous for their swimming abilities. They're not native, and it's theorized that the island was meant as a food store for some crew of sailors and that they planned to eat the pigs. In any case, it's entirely possible that this could be the beginning of a new clade of marine mammals.
Perhaps have your sea boars be actual pigs (which are domesticated boars) which colonized the ocean - whales are an example of what aquatic artiodactyls would look like in reality, so you could go for a Cetacean-like body plan but have the boar-like features retained, or perhaps just evolved secondarily.