I’ve always thought of ichthyosaurs as dinosaurs. But technically, I gather they’re classed as ancient marine reptiles while dinosaurs were those ancient reptiles that lived on land. Wikipedia highlights this issue more. Ichthyosaurs, of course, could swim. Since they aren’t dinosaurs, could dinosaurs swim?

Swimming dinosaurs touched bottom of lake from Reuters says yes, certain types. Footprints found on the bottom of a lake give evidence that some swam (and scraped the lake bed as they moved along).