Thanks to Othneil Charles Marsh, we have an accurate reconstruction of the huge 14 metre long Elasmosaurus - last and largest of the Plesiosauroid Plesiosaurs. There is a similar species of almost the same size as Elasmosaurus called, Thalassomedon, from a slightly earlier part of the Late Cretaceous (the two cousins probably met)
Elasmosaurus didn't appear in the main Walking With Dinosaurs series.
Elasmosaurus was one of the hazards in Hell's Aquarium (Nigel's nickname for the Most Dangerous Sea of all time), in Sea Monsters: A Walking with Dinosaurs Trilogy Even so, they seem to be benign and passive creatures, gently migrating through the sea; however, this could be due to their task of migrating and nothing else. Still, as their primary prey is small fish, there is no apparent reason for them to attack the humans (though one is seen to investigate the ROV Camera with its jaws open)