And might be there were no vacant niches in the oceans at all. Yet land mammals pushed back all species from their "local maximums" there. Might be that on the land there were much more emtied niches as in the ocean at that time and yet the evolution was driven in some cases by internal forces to the sea whatever it cost.
Anyway I have never heard about adaptive radiation of water species during the period discussed. The greatest adaptive radiation in the ocean I know about occured during cambrian explosion. So preliminary I would not bet a nickel on it.
Then you would be wise as you would lose your nickle, sorry. The K-T event devastaed shallow water marine ecosystems. The ammonites, huge predatory cephalopds (coiled shells 3 meters across) were extremely widespread and diverse. These probably had a top-down control on prey items (IOW Cretaceous bad time to start evolving from terrestrial to aquatic). Belemites, another group of cephalopods were also extinct after this event. Rudists were very large bivalves (not closely related to any living bivalves) that formed massive reefs in Cretaceous seas and kilo for kilo probably accounted for most of the aquatic biomass in shallow seas. Completely absent after the Creataceous.
Echinoids (sea urchins and their kin) lost pretty much all of their diversity as did brachiopods. Large numbers of fish families became extinct. Marine phytoplankton groups like diatoms and coccolithophores crashed in both numbers and diversity. In the large vertebrate world (those that might have occupied niches to be later filled by pinnipeds and cetaceans) all of the marine reptiles went extinct. This, btw, includes the marine crocodylians. The group including modern marine crocodiles are not known until the Tertiary so your statement in another post about no change in crocodile diversity is, well, a crock (just an aside, and maybe this was an error, but you might want to avoid citing undergraduate web projects as primary sources. Especially when those sources do not contain the info you claim anyhow).
Nearly all of our shallow water marine mollusks and echindoderms are families which diversified since the K-T event. Decapod crustaceans (crabs, lobsters, shrimp) evolved prior to the event but exploded in diversity during the Tertiary.
Edited by Lithodid-Man, : Forgot to add what did diversify during the Tertiary!
Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"