Scientists aren’t entirely sure what caused the Cretaceous-Tertiary extinction, asteroids, volcanic activity, sea level change, and climate change have all been hypothesized. Any way you look at it, it was not the impact of an asteroid or the eruption of the volcanoes that caused the extinction, it was the change to the ecology of the planet in the wake of that event. Mammals, birds, reptiles other than dinosaurs, and other animals had already evolved prior to the extinction. These animals, particularly mammals, were better suited to the ecology of the planet after whatever event precipitated the extinction. They were more flexible and more adaptable, hence they survived while the dinosaurs died. That’s why we still have animals.
As to water dinosaurs, since the event that cause the extinction did so by altering such things as sea levels, global temperatures, and the amount of sunlight available for photosynthesis, there is no reason to assume that being aquatic would have offered much protection. There is, however, a remote possibility that some aquatic dinosaurs survived, but there are a number of reasons for assuming that they do not:
1. Despite great effort, no reliable evidence has been found for the existence of a Loch Ness Monster.
2. We’ve explored even the deepest parts of the ocean now, fisherman have been catching fish all over the planet for centuries, and myriad strange and unusual sea creatures have turned up in nets and on beaches, including some fish once though long extinct, but no dinosaurs, not even remnants in shark stomachs (plesiosaurs ranged in size from 3 to 20 meters, so some would have been edible to sharks).
3. The continents have drifted and sea levels changed since the Cretaceous period. Areas that were underwater then are dry land now. We would expect to find newer fossils of aquatic dinosaurs if some had survived the extinction.