LIOPLEURODON
If Jurassic Park had an aquarium scene, and actually featured more animals from the Jurassic period, liopleurodon probably would have been in it. Although the actual length of these beasts is contested (some scientists have claimed lengths in excess of 50’), most agree that it was around 20 feet in length, with a full fifth of that being pointy-toothed head. When the mouth of the “smaller” estimate is still plenty large to eat you whole, I think that is perfectly huge enough.
Scientists have tested the paddle design of these creatures on small swimming robots and found that although they would not have been incredibly fast, they were incredibly agile. They also would have been able to make short, fast burst attacks like crocodiles, which in no way makes them any less intimidating.