Quoting Mike Taylor <mike@indexdata.com>:
It's always possible, but since marine animals generally have better preservation potential than terrestrial, it seems unlikely. We've found enough ichthyosaurs, plesiosaurs, nothosaurs and mosasaurs by now that it's hard to believe a robust clade of aquatic NON-AVIAN dinosaurs would have remained undetected till now.
-- Nick Pharris Department of Linguistics University of Michigan
"Creativity is the sudden cessation of stupidity." --Edwin H. Land