From the roughest seas to the crystal clear waters of the Florida springs, David Attenborough swims with sea otters and dives with manatees, as he follows those mammals who, millions of years ago, left dry land and returned to the water to feed.Attenborough races across the Pacific Ocean to find the largest mammal that has ever lived on this planet, the blue whale, a hundred feet long. As David says, 'nothing like that can grow on land because no bone is strong enough to support such bulk. Only in the sea can you get such huge size as this magnificent creature'. He also bounces through the waves off New Zealand to witness an enormous pod of high-speed dolphins pursuing their fish dinner.Although some marine mammals like seals and sea lions still come ashore to breed, all porpoises, dolphins and whales have evolved to court, mate and give birth in the water. Indeed the sight of humpback whales mating is truly amazing, with the males wielding the longest penis in the animal kingdom - twelve feet long - and so highly mobile that it can seek out the female genital opening as she swims alongside.