Marill, despite being a water-type pokemon, belong to the same evolutionary family as Plusle, Minun, Pachirisu, and Pikachu. They are recent arrivals to the world of pokemon, a creation of the Pleistocene contemporaneous with humans. Specimens of a pokemon blue in color and somewhere between it and Pikachu in shape have been found in glacial ice and dubbed "Pikablu" by the media, a name occasionally and erroneously applied even to true Marill.
Although modern Pikachu are poor swimmers, their ancestors were amphibious, flying creatures who hunted with electricity and preyed on fish in the manner of Chinchou and Lanturn. At times even today a Pikachu is found who has not lost the ability to surf or fly, albeit never both, and Marill descend from these surfing Pikachu. Their color changed to allow for camouflage in water, their body shape became an extremely buoyant one, their tail filled with oil and can even be used as a flotation device, making them need little energy to swim. Even their diet changed dramatically. Pikachu are insectivores, although their ancestors supplemented their diet with small fish and birds. Marill, however, are completely herbivorous, a shift so dramatic it has caused some to wonder whether Marill is merely a case of convergent evolution.
But evolution can work quickly in desperate times, for the ocean in which Marill were born was made so barren by the cold that there were no fish or Surskit left to eat, only seaweed. Although their ancestors vanished with their island home, the Marill, changed enormously, survived.