Liopleurodon
These large, carnivorous reptiles would’ve devoured us alive. Luckily, these now extinct animals no longer roam the seas, so we don’t have to worry while swimming in the ocean. They became extinct at the beginning of the Cretaceous Period, and were pretty deadly to other animals around them. It wasn’t until 150 million years ago when they went extinct, because a more vicious reptile emerged, the mosasaur, which ended up destroying the Liopleurodon (History of Yesterday).