That's just the beginning.
Also, should the Loch Ness Monster really exist, it probably wouldn't fair too well in such chilly water. In all likelihood, it would prefer much warmer climates (the tropics, for example).
The real problem with the creature's existence is air. A creature that big would need to come up for air...a lot. Were this the case...and especially if there were a colony of them...not only would sightings be far more common, it would actually be impossible to look at the Loch WITHOUT seeing a monster.
However...
I still don't want to rule out other possibilities, or to conclude that undiscovered lake/ocean monsters do not exist at all. Who knows? Could be SOMEthing out there, in Loch Ness or elsewhere.
Maybe it's the ghost of a dinosaur?