It was always one of the cardinal rules when I was younger: when shown a mosasaur or plesiosaur labeled as a dinosaur, one could quickly refute it by saying that dinosaurs were not aquatic. It missed the point, perhaps, but it was one pithy characteristic to keep at the ready.
Of course, that story is much more complicated now that the dinosaurian origin of birds is recognized, and the line between "dinosaur" and "bird" gets so fuzzy as to be absurd. As Craig Dylke pointed out recently, there certainly were marine dinosaurs, with Hesperornis as a good example.
Continuing its 2012 lecture series, the Royal Tyrrell Museum's YouTube channel recently shared a lecture from Joe Sanchez covering the hesperornithiforme fossil record in Saskatchewan. "Diving Birds in the Prairies: Late Cretaceous Hesperornithiformes" covers a general overview of the family as well as highlighting the bonebeds in Canada which have given us the oldest North American hesperornithiforme fossils, dating to about 95 million years ago. These fossils represent the genus Pasquiaornis, a basal member of the clade, and Sanchez demonstrates the way that its anatomy is transitional between wing-propelled divers and foot propelled divers like the famous Hesperornis. Enjoy!