The marine reptile with a flattened, paddle-shaped tail that lives mainly in the Indian and Pacific Oceans is the sea snake.
Sea snakes are venomous elapid snakes adapted to life in warm coastal seas. Their laterally compressed bodies and paddle-like tails help them move efficiently through water, while many species have nostrils positioned high on the snout and can remain submerged for long periods.
Most sea snakes occur in tropical waters from the Indian Ocean to the western Pacific. They are not sea turtles or marine iguanas: sea turtles are shelled reptiles, marine iguanas are lizards restricted naturally to the Galápagos, and crocodiles retain a more conventional, muscular tail shape.
The group includes true sea snakes and sea kraits. Most true sea snakes are unable to move effectively on land, whereas sea kraits can come ashore. One especially wide-ranging species, the yellow-bellied sea snake, crosses much of the tropical Indian and Pacific Oceans but is absent from the Atlantic.