Do cetaceans only live in salt water environments?

No, some cetaceans, such as the Amazon river dolphin (Inia geoffrensis), the La Plata dolphin (Pontoporia blainvillei), the Ganges river dolphin (Platanista gangetica), and the Indus river dolphin (Platanista gangetica minor), live in freshwater environments, such as rivers and estuaries.