Fraggle Punk
I didn't know much about megalodon before responding to this. I just wikipedia'd it. It says that megalodon lived until about 1.5MM years ago which is relatively recently on a geological scale. Everyone is familiar with the tale of the Coelecanth, which was thought to have been extinct more than 65MM years ago. So I think it would be more plausible to find an extant megalodon somewhere than some of the other far out theories about dinosaurs still surviving in Loch Ness or the Congo. It's still far out, though. The big drawback to the idea is that where the Coelecanth is a nice small fish that can hide in deep ocean cravasses, the megalodon is huge, and would likely need to feed on other large animals. It's hard to imagine that with the state of our overfished oceans nowadays, that someone wouldn't have noticed some signs of such a creature. But it could be possible.
In Wikipedia, megalodon is classified in the genus Carcharodon, the same as the great white, but there's an unexplained note saying that this classification is disputed.
But it does raise the question of whether the great white is a direct decendant in a continuous line -- basically the megalodon which has merely decreased in size in adapting to changing climates.
This is probably the most likely scenario; while it's possible that there could have been one around like, just a few hundred years ago, whaling has reduced a once couple million population to like only a few thousand, probably killing the shark and a large portion of giant squids off with it.
I think it would be very interesting if the great white shark is really just an ancient predator, like the alligator, simply adapting to a new environment; supposedly, the megaladon is just like a great white shark.