With very little of the ocean explored, I would think it's safe to assume that there are several organism species yet to be discovered, and perhaps prehistoric species once thought to be extinct still existent today(the coelacanth is an exceptional example). Myths and legends are no exception; however, I am sure the most plesiosaurs dwell within the ocean, though living things adapt(some not as well as others), and if the Loch Ness monster was existent, this would be a good reason why.
It would be quite exciting to see something such as a liopleurodon today. However, I'm most likely be going ahead of myself a tad too much.