How about marine dinosaurs?

*reptiles

Technically the only aquatic dinosaur linage are the spinosaurids. All other aquatic reptiles are not dinosaurs in the strict sense because pleisosaurs and pliosaurs (including Liopleurodon) are more closely related to turtles than dinosaurs and the mosasaurs is the furthest away from being a dinosaur proper since mosasaurs are lepidosaurs rather than archosaurs like dinosaurs. All remaining prehistoric aquatic reptiles are either  archosauromorphs, lepidosauromorphs, primitive diapsid reptiles, or parareptilians.