These giant sea reptiles lived in freshwater rivers, too (earthsky.org)

[eppur_se_muova]

8. Mosasaurs, Plesiosaurs, & Ichthyosaurs were not dinosaurs, but separate lineages.

So too were pterosaurs.

But your point is still an interesting one.

The extinction event wasn't selective for dinosaurs, but apparently for all larger megafauna, particularly on land.

The only large survivors were some crocodilians, turtles, sharks, and apparently giant squid. All were at least partially aquatic, and the reptiles were cold-blooded -- both can estivate for many months (crocs in burrows, turtles in muddy bottoms without even coming up for air). It seems large, active, high-metabolism (i.e. needing high food input) animals unprotected by large volumes of water were vulnerable. Fully aquatic (non-air breathing) animals such as sharks would have had the advantage of being able to swim away from the most affected areas -- even swimming as far as the antipodes, as some large sharks and whales do today. And benthic animals, such as giant squid, would have been most protected of all.

Currently, mosasaurs and ichthyosaurs are believed to have been warm-blooded, so apparently unable to survive the post-Chicxulub environment.