Hint : waters were already throughly and intensively occupied with successful clades of Reptiles beforehand.

• Ichthyosauria | Ichthyosauriformes

• Pistosauroidea | Plesiosaurs ( Including : Pliosaurus as well )

• Mosasauria | The Mosasaurs

Not to mention Sharks were already thriving and growing into Massive Sizes.

Even several Fish evolved into colossal sizes throughout the Jurassic and Cretaceous despite competition