http://www.innvista.com/science/zoology/dinosaurs/sea.htm
http://en.wikipedia.org/wiki/Plesiosaur
http://www.oceansofkansas.com/plesiosaur.html
So, some dinos at least in the often used sense of the word, if not modern preferred classification jargon, were water dinos. Others used to be considered so, but they now proclaim them things like maybe 'forest' dwellers. "
"Palaeontologists in the 19th Century assumed that giant sauropods like the long-necked brontosaur must have lived in water because they were as big as whales. "
http://news.bbc.co.uk/1/hi/sci/tech/1470305.stm
In the pre split world, and pre flood, there was lots of water around, and the assumptions need to be revisited, because they are PO assumptions!
That their body weight would make them sink in the mud, etc. so maybe they weren't near water after all..etc.
If some dinos were largely water creatures, maybe they could have survived a flood?
But, either way, we need to simply look at the evidence here. Was Loch Ness really a 'dino'? These other supposed sightings at sea, if real, may indicate some survived the flood. What about the dino in Job? That " 41:
31 He maketh the deep to boil like a pot: "
Just because science lags in the knowing much about it department, doesn't mean such things were not so at all.