I managed to find the exact stretch of beach that the walruses are shown hauled out on in Google Maps , which confirms that these shots were filmed at Point Lay in Alaska, which is known for this phenomenon. [Link 1]
Hudson Bay, Canada
This was shot in Hudon Bay, Canada, according to Assistant Producer Elizabeth White . [Link 1]
Cape Crozier, Antarctica
Despite the narration telling us about the increasing prevalence of killer whales in the Arctic, at least two of the four shots used here are from the earlier episode "Spring", in which a pod of Antarctic killer whales travel through Cape Crozier. In that sequence, the whales seen are the "Type C" fish-specialist killers, visually distinguished by their forward-slanted eye patches. I assume that the filmmakers didn't specifically film killer whales in the Arctic and re-used shots from Antarctic filming to complete the narrative here.