This is where I get boring. I WARNED YOU ABOUT THIS!
The author G.K. Chesterton once said that people have been hung on less evidence than there is for the Loch Ness Monster. Strange things have been sighted there for centuries. (This is why it's ridiculous to talk about ONE animal). Over the years, there have been liars, hoaxers and, quite simply, those who have mistaken something perfectly normal for something they WANTED to see. But that doesn't explain the hundreds of rational people--including monks, doctors, experienced locals and respected professionals---who swear they've witnessed something extraordinary in those waters. Loch Ness is 20 odd miles long, 2 miles wide and, at certain points, deeper than than the North Sea. I'm certainly not going to say that it's impossible for some strange animals to be swimming around in there. In 1969, I was driving with friends on the North shore when we saw something weird in the loch. This "thing" disappeared below the surface in seconds and, for all I know, was a trick of the light. We were shaken enough to stop the car at the first opportunity and ask some tourists if they'd seen anything. They hadn't.
I don't know if the Loch Ness Monster exists but I would certainly never say that it doesn't.