When you look out at the white-caps on a wind-swept lake, you can see a dark, undulating pattern under the crests of the white-caps.
Could this shadow-like area explain the sightings? Revised, see below.
Edit First of all, it must be a rogue wave, , so it stands out from the rest of the waves (if any). This could be the hump of some monster just breaking the surface!
Edit You are walking along a country lane, when a sudden motion on the water catches your eye. Followed by the sound of a crashing wave. Within seconds, only ripples remain on the surface. You remember the image as shown above. It could be a white-cap on a wave, but everyone who has been to the beach knows that waves come in one-after-another, and there are no other waves, so it can't be a wave. You notice the darker area under the white-cap and realize the only possible explanation is: you have just seen a large animal dive under the surface. Like a sounding whale. (Or monster?).
Loch Ness is located near a major fault line that has seismic activity. Physicists can explain that a wave created by seismic activity is a rogue wave or tsunami. They can also explain why the wave does not reach shore and, why the area under the white-cap is always darker.
So, could a collaboration between physics and psychology show this illusion is a possible explanation for Nessi?