This episode concentrates on marine dwellers. Astride an elephant, Attenborough highlights their love of water, before moving on to those that are completely at home in it. In proportion to their size, sea otters probably have the biggest appetites of any mammal, and Attenborough swims with them off the Californian coast. Their adaptations include webbed feet, which in one way or another (as flippers) are common to all sea-going mammals. Sea lions are shown leading their young into the water for the first time, and navigating entangling beds of kelp. In Antarctica, the differences between true seals and sea lions are illustrated: the former don't have the external ears or the mobility on land of the latter. Meanwhile, in the Arctic, ringed seal pups are prey to polar bears. Other pinnipeds shown include hooded seals and harbour seals. The manatee is a grazer descended from land-living herbivores and spends its entire life in the water. Near south-eastern America, there live dolphins that specialise in synchronically 'herding' fish on to the river banks before feeding. With the aid of computer animation, Attenborough walks the length of a blue whale — the largest creature on the planet — to demonstrate its vast physiology, and then travels alongside one in the open ocean. Whale song, and particularly that of humpback whales, is examined. The tumultuous breeding habits of southern right whales are shown off the shores of Patagonia.
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