I see that many people are saying that they spawn near carno island, they also spawn near herbivore island
Mosasaurus
An elusive marine lizard and reputed as one of the largest creature found in the waters, the Mosasaurus is much more powerful than any known aquatic beast encountered in the depth. While its sluggish speed may put off some survivors, being ambushed by one can mean a certain death.
How to survive, fight, and kill a wild Mosasaurus, from players who have done it.