IIRC, the "real" Nessie was uncovered a few decades back. Before paleontology made the dinosaurs cool and Nessie got its pleisiosaur retcon, it was described as a boiling patch of turbulence on the lake's surface. The actual monster was never seen but was presumably some sort of faery or other supernatural beastie.
The actual "monster" is a product of the fact that Loch Ness is a long, thin loch prone to frequent wind storms. When the wind blows down the long axis of the loch all of the warm water near the surface gets pushed to one end of the loch. When the wind stops the upper surface of this warm water wedge relaxes fairly rapidly. However, the lower surface is a boundary between warm water and cold water, so it relaxes much more slowly, forming a body wave that travels the length of the loch in a few days. Nobody would notice it except that part way down the loch there's a shallow ridge that forces the body wave to the surface. The end result is that a couple days after a strong wind you can be out on the loch in very still conditions and suddenly find yourself in weirdly unexpected turbulence that has no obvious cause. It's easy to see how some superstitious fishermen might have jumped from that to a lake monster.
The actual "monster" is a product of the fact that Loch Ness is a long, thin loch prone to frequent wind storms. When the wind blows down the long axis of the loch all of the warm water near the surface gets pushed to one end of the loch. When the wind stops the upper surface of this warm water wedge relaxes fairly rapidly. However, the lower surface is a boundary between warm water and cold water, so it relaxes much more slowly, forming a body wave that travels the length of the loch in a few days. Nobody would notice it except that part way down the loch there's a shallow ridge that forces the body wave to the surface. The end result is that a couple days after a strong wind you can be out on the loch in very still conditions and suddenly find yourself in weirdly unexpected turbulence that has no obvious cause. It's easy to see how some superstitious fishermen might have jumped from that to a lake monster.
