Funny enough, some animals went back to the seas - whales, dolphins, etc....
Whales and dolphins are cetaceans, all of whom have a single ancestor. They are descended from artiodactyls, the even-toed ungulates. Probably from a primitive hippopotamus-like creature who swam all the way to the mouth of the river and liked it there.
Warm-blooded air-breathing animals (mammals and birds) have much more powerful metabolisms than cold-blooded and/or gill-breathing animals because they have more oxygen at their disposal. Mammals who have re-adapted to aquatic life virtually rule their ecosystem. Dolphins can defeat or evade all but the largest predators, and they can even handle them because they are pack-social and work as a team. Polar bears are invincible in the water, although I don't know who wins when they enounter an orca (the largest dolphin species). Perhaps they don't swim in the same waters.
The pinnipeds (seals, sea lions, walruses, etc.) are, like the polar bear, not completely aquatic (they sleep and reproduce on land), but it takes a giant predator to catch them in the water. The pinnipeds are descended from one common ancestor, a member of the order Carnivora and probably a primitive bear.
River-dwellers like otters (also carnivorans, members of the weasel family) dominate their ecosystem.
However, nothing in nature is 100%. The manatee is a grazer, not a predator.
Penguins are near-fully adapted to marine life, but have to sleep and reproduce on land. They dominate the fish in their ecosystem, but larger mammals like orcas and seals eat them.