The 'proof' behind the Loch Ness Monster is a picture, in this case claiming to show the creature's long neck and head poking out of the water, is worth more than a thousand words, even after it is revealed by its snapper to be a hoax.
The preferred explanation is that Nessie is a plesiosaur, a remainder of the Mesozoic era who has oddly dodged the need for a breeding colony. The legend serves more as a bedtime story as understanding of marine life has become more refined.