The first monster tale comes from the time of St Columba, whose loud voice was said to have scared off a large fish that was attacking one of his monks in the river Ness, that flows from the loch into the North Sea. That was in the sixth century, then there was nothing until the 1930s, when one dark night some motorists spotted a large creature crossing the lochside road. I think that it was likely to be a large red deer stag seen in the interplay of headlights and shadow. But the press got hold of the tory, dubbed the beast "the monster" and thus was a legend born, along with several hoaxes. All pictures of the beast are hoaxes, but some unexplained wakes have been seen.
There have been genuine mistakes. One is caused when a motor boat speeds down the narrow loch and its wake bounces off both sides and the two wakes meet in the middle. A line of waves appears to move down the loch and it is sometimes mistaken for the dorsal fins of a large fish.
But the problem comes with the word monster, which suggests an exotic beast unknown to science. There is something down there, but it is not a monster. There are three possibilities.
a: Sturgeon. This large fish was once common in Scottish lakes. Did a colony of them survive? However, while sea sturgeon have been found in the Moray Firth, which is connected to the loch by the river Ness, none has been caught or sighted in the loch, and no sturgeon DNA has been found.
b: Catfish. Once these fish were released into the loch, but no catfish DNA has been discovered.
c: But eel DNA has been found aplenty in the loch, the species being the European eel. Moreover, with no predators in the loch there is nothing to prevent eels growing large. Large, though not giant, eels are reported in other lakes, such as Loch Morar, which is very close to the sea, and some Irish loughs [lochs.] The monster is just an eel grown large.
Talk of monsters is therefore misleading. Keep the monster story for cartoons.