Water Creature Extinctions?

J, I understand the flood and how it affected our animal population and extinction of dino's and such, but what about water creatures that died off? They didn't drown! What happened to megladon? and all the other various water dino's? Could it be that some of these very creatures still exist giving us Loch Ness and other legends? Ben in FL (my Dorito-stealin’, blue-eyed, Dark Irish, Christian brother – and I still have that sweater you gave me like 4 years ago – my wife says it looks great, so it's a keeper. But I never could fit into any low-cut loose-boot jeans, as you recommended ... 'guess I'm too old for that sorta thing!)