whale
A huge extinct fish.
30 meters
It weighed about 50 tons...
72 feet (22 meters)
It is the Leedsichthys. It is over 90 ft.
ok this is it from 10 to 1(this is close maybe not acurate but close) 10.basilosaures 9.diplodicus 8.supersaures 7.sismosaures 6.brachiosaures 5.argentinasaures 4.giant mosasaur 3.liopleurodon 2.leedsichthys 1.blue whale
Above is a link to a scale representation of prehistoric sea creatures. For the looks of it Leedsichthys seems to be the biggest sea creature. Megalodon was estimated to be around 33 feet in length. The Liopluredon was an estimated 25 feet. The Sarcosuchus was estimated to be 37-40 feet. Thus leaving the Leedsichthys, Leedsichthys Problematicus was a giant pachycormid (an extinct group of bony fish) that lived in the oceans of the late Jurassic period, 165-155 million years ago. Only partial skeletons have been discovered, so its exact size is uncertain. Estimates range from 15 to 30 meters, but the best estimates believe it to have grown to around 20-22m (= 66 to 72 feet) long, making it the largest fish ever to exist. Hopefully this helps. By no means is this answer absolute I am not educated in Prehistoric Sea Creatures, it's just data gathered from searching the Internet, as well as various National Georgraphic sites I have come across. There is a program that is hosted by Nigel Marvin called Prehistoric Sea creatures that touches upon all creatures listed above.---The Megalodon has actually been recorded to be 55-60 feet in length, I also found this out from a Nat Geo site. I am also not a "Prehistoric Sea Creature" specialist.. but I've looked everywhere on the internet to find the definite world's largest. Just to put in "Predator X" or "Liopleurodon" measured up to 50 feet in length.
There are a couple problems with your question seeing as swimming dinosaurs didn't exist, and I'm not sure what you mean by gigantic world, but I will do my best to answer it. The largest aquatic reptile from the time of the dinosaurs was a carnivirous pliosaur 15m long. It is unamed. The largest ocean going creature at the time was Leedsichthys, a massive fish 27m long.
The biggest fish was the Leedsicthys, about the size of a baleen whale.
because it is big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great and big and great.
No.
big