Which of these lives in the sea?

A dolphin is any one of the 40 extant species of aquatic mammal from the cetacean families Delphinidae, Platanistidae, Iniidae, Pontoporiidae, and the probably extinct Lipotidae. All these families belong to the parvorder Odontoceti, i.e., toothed whales, which also include the closely related families Monodontidae and Phocoenidae (porpoises), as well as the more distant families Physeteroidea and Ziphiidae.