Well, if the Ceolocanth was deamed extinct and then was found alive, I believe that there might be more prehistoric creatures in the ocean. We have of yet to go to the bottum of the ocean yet. I think

They would've evolved so they probably wouldn't look like they did back then. I heard the Loch ness monster could've been one too.

Pleisosaurus

There are some who think that there is evidence that Megalodon is still alive under there. Here's a thread about it.

There is a slight possibility...but its absolutely impossible for a dino to be living at the bottom of the oceans....Dinosaurs are not marine animals

None of the well known prehistoric sea reptiles were dinosaurs or close relatives of dinoaurs. Most are more closely related to modern lizards, though this has no bearing on whether prehistoric type marine reptiles could still exist.

I just had to point out his mistake of saying "dinosaur"