Mike
I did a quick web search. The ichthyosaurs were already extinct well before the end of the Cretacious:
Ichthyosaurs swam in the Mesozoic ocean when dinosaurs walked on land. To be precise, they appeared slightly earlier than dinosaurs (250 million years ago versus 230 ma) and disappeared again earlier (90 ma versus 65 ma).
They, and the plesiosaurs, were aquatic reptiles, not dinosaurs (there is a bit of a difference). The plesiosaurs evidently lasted right up to the end of the Cretacious. It is possible that a general collapse of the aquatic ecology caused by the die-off of algae and the itty-bitty critters that lived on it and cascading up the food chain, would have killed off these top-niche predators. Sharks survived, but they weren't necessarily large animals. The megaladon comes much later in the fossil record.