What was the largest aquatic reptile?

The present day saltwater crocodile can reach lengths of 6 meters (20 feet) comparded to the other fearsome reptile, the Komodo dragons 2 to 3 metres (6.6 to 9.8 ft), the croc comes out an easy winner. When prehistoric animals are considered, Deinosuchus (an ancient crocodile), measuring up to 12 m (40 ft) and weighing up to 8.5 metric tons beats out all it modern relatives.
But beating all comers was Kronosaurus (12.8 m long (42 ft) long). It was probably the largest marine reptile that ever lived. It was a plesiOSaur, a meat-eating marine reptile living in the Late Cretaceous Period. Similar sized snakes prowled the Amazon area back in those days as well.
Do not confuse dinosaur s with repiles hover. These grew much bigger but were a different species entirely.