Behold the dugong (Dugong dugon). It's the only strictly marine herbivorous mammal. They feast primarily on sea grass and are particular about what fields they feed upon. Related to both the manatee and elephants, they have forsaken their hind limbs in favor of a whale-like tail and forepaws in favor of paddles. They've also forsaken comeliness in favor of a vacuum-style mouth.
They are supposedly the source of many mermaid myths. I imagine those confused sailors either need to have their eyes checked, or maybe they just spent too much time at sea.
Photo source: Webecoist.com