http://www.mce.k12tn.net/dinosaurs/elasmosaurus.htm
Maybe this guy is the culprit? At least he lived in the late Cretaeceous. The other guy people confuse Nessie with,
http://www.prehistory.com/plesiosa.htm - the plesiosaurus, from who elasmosaurus undoubtedly evolved, died out at the end of the Jurassic period. And he was a lot smaller and less intimidating. Plesiosaurus grew to be a pretty puny 17 feet long while Elasmosaurus grew to be 45-50 feet long.
It's unlikey that any of them made it through the K-T extinction, most likely caused by the dual impact of the Chicxulub and Shiva cratermakers, at the tip of the Yucatan peninsula of Mexico (120 miles wide, 1 mile deep) and under the Arabian Sea, near Bombay, India (370 miles wide, 7.5 miles deep), respectively.
Around here we have tales of lake monsters,
http://www.kidzworld.com/site/p2206.htm "The Champ" -
http://www.angelfire.com/home/lake/champ.html
National Geographic came to the conclusion that they weren't monsters, but sturgeon. Sturgeon are a pretty big and scary monster all by themselves
http://worldstar.com/~dlarson/