Seel/Dewgong, Marril/Azumarril, Suicune, Wailmer/Wailord, and Speal/Sealeo/Walrein all are mammalian Water Types :/
You're absolutely right, it should have occured to me. But the fact that it lacks the traditional blue coloring of a water pokemon or the fish-like shape of a water pokemon kind of set it apart from most water pokemon.