Polar bears are considered marine mammals
Polar bears are a special case among bear species. They're adapted for life on the ice, hunting seals and scavenging whale carcasses. Their life in the Arctic has fine-tuned their features such that they're actually they only bear species to be considered a marine mammal.
They depend on the ocean for their food and for their icy habitat. Thus, they are considered marine mammals and fall under the Marine Mammal Protection Act.
Because polar bears depend on the ocean for food and a place to live, they are particularly adapted to an icy and watery environment. Polar bears have partially webbed forepaws that help them speed through the water at speeds reaching six miles per hour, helping them to navigate from ice floe to ice floe. Their layer of blubber and thick coat provide buoyancy and protection from the cold water, and their nostrils close when under water. All these adaptations and more help them to hunt seals and thrive in what seems like a barren landscape.
All this also makes sense when you consider that pinnipeds — seals, sea lions and walruses — are actually the closest relative to bears on the evolutionary tree!