Its a wave....i have seen these before on large lakes and it can often fool people.If you have evr seen the real thing like i have the body or back hump appendage comes about 1 to 2 feet above the water.
it could be a whale, or a fish. or a shadow. but i think you have a good point; Scotland is home of many weird Nessie stories. like the one about Nessie and how she lives there and its been proven and debunked