Dugong

The dugong, or sea cow, is a large, grey-brown marine mammal with a torpedo-shaped body, no dorsal fin and no hind limbs. Its front limbs are short, paddle-like flippers, and it swims with a notched, dolphin-like tail fluke, which tells it apart from the manatees. The snout is sharply turned down, an adaptation for feeding on seagrass on the seabed. Males grow a pair of short tusks at puberty. Its closest modern relative, Steller's sea cow, was hunted to extinction in the 18th century (Wikipedia, general species account).

The dugong is the only strictly marine plant-eating mammal. Across its range it depends entirely on seagrass meadows, which limits it to warm, shallow coastal waters, especially wide, sheltered bays and mangrove channels; it can tolerate brackish water. Populations throughout its range have been reduced by hunting for meat and oil, drowning in fishing nets and loss of seagrass, and many are now small and isolated (Wikipedia, general species account).