Id say its possible but not likely. Ive read that sonar records found a large moving object in the mariana trenches the object was close to 80 feet. I cant be certain on this as you can post anything to the internet. Great whites are relatives of the megalodon and are still here but only will reach maybe 25-30 feet. the mariana trench would be a great hiding spot for such a massive animal, but i cant imagine there would be enough food at such a depth for a breeding megalodon population. Then again it was pretty recently that scientists discovered the coelecanth which was believed to be extinct for over 70 million years. If an animal like that still exists today, i wouldnt doubt other prehistoric species are out there, its just a matter of being in the right place at the right time with a camera.