Walking with Dinosaurs (1999)

S1E3 Cruel Sea

While dinosaurs reign on land, the oceans tell a different tale. Marine reptiles command the waters. A group of female Opthalmasaurus is heading to the northern Tethys Sea's reefs to give birth. Once the young ichthyosaurs arrive, they must navigate a perilous world filled with threats, including sharks, the plesiosaur Cryptoclydus, and the fearsome pliosaur Liopleurodon, the largest predator of its time.