Polar bears

The polar bear is the most carnivorous member of the bear family, and the one that is most likely to prey on humans as food. It feeds mainly on seals, especially ringed seals that poke holes in the ice to breathe, but will eat anything it can kill: birds, rodents, shellfish, crabs, beluga whales, young walruses, occasionally musk oxen or reindeer, and very occasionally other polar bears. Still, reindeer and musk oxen can easily outrun a polar bear, and polar bears overheat quickly: thus the polar bear subsists almost entirely on live seals and walrus calves, or on the carcasses of dead adult walruses or whales. They are enormously powerful predators, but they rarely kill adult walruses, which are twice the polar bear's weight, although this has been known to happen.[22] Humans and larger bears of their own species are the only predators of polar bears.
As a carnivore which feeds largely upon fish-eating carnivores, the polar bear ingests large amounts of vitamin A, which is stored in their livers; in the past, humans have been poisoned by eating the livers of polar bears.[23] Though mostly carnivorous, they sometimes eat berries, roots, and kelp in the late summer.

Polar bear diving in a zoo.
Polar bears are excellent swimmers and have been seen in open Arctic waters as far as 60 miles from land. In some cases they spend half their time on ice floes. Their 12 cm (5 in) layer of fat adds buoyancy in addition to insulating them from the cold. Recently, polar bears in the Arctic have undertaken longer than usual swims to find prey, resulting in four recorded drownings in the unusually large ice pack regression of 2005.[24]
Polar bears are enormous, aggressive, curious, and potentially dangerous to humans. Wild polar bears, unlike most other bears, are barely habituated to people and will quickly size up any animal they encounter as potential prey.
Like other bear species, they have developed a liking for garbage as a result of human encroachment. For example, the dump in Churchill, Manitoba is frequently scavenged by polar bears, who have been observed eating, among other things, grease and motor oil.