If you read The Great Orm of Loch Ness by FW Holiday, you'll find references to depressions and areas of crushed foliage that, appartantly, the locals attribute to Nessie. Loads of people think Holiday's a bit of a nut, but his books are definately worth a read. I'm a bit of a fan myself.
RE: the plesiosaurs. It really is not plausable that any animal that has to breathe air and is the size that Nessie apparantly is, could remain undetected in a body of water that small. I know Loch Ness is large compared to, say, your garden pond, but on a global scale it's not very big at all. Let's say Nessie as a lung capacity on a par with a sperm whale and can go without air for about an hour. That means at least once an hour, an animal is going to have to show itself on the surface. Presumably, there's a whole colony of creatures and not just one individual, so that's a number of very large creatures that would need to surface at least 24 times a day each. On a confined, narrow body of water with the amount of surface traffic the Ness has, there's no way that could remain undiscovered. And besides, sometimes months and years elapse and no one reports a sighting at all.
Someone's going to point out that plesiosaurs etc can use their long necks like a snorkel to breath on the surface undetected by only showing their nostrils above water. A similar thing was once believed about sauropod dinosaurs, brachiosaur et al. THe theory was discounted though, because it doesn't work in practise. More than a few feet below the surface, the water pressure is too great for the animal to draw a breath. An animal would have to lie with its lungs very close to the surface in order to breathe, again making it very visible on a regular basis.
Most of the evidence suggests long-necked plesiosaurs were primarily surface dwellers anyway. We know the angle they held their flippers at prevented them from diving, and their long neck would be very cumbersome under water. They probably used their necks like water birds do, fishing from the surface, resembling very much a giant reptilian swan.
Pretty damn spectacular on the end of a fishing line. but not in the Ness