Here's a site from the anti-Nessie-is-a-plesiosaur folkshttp://www.plesiosaur.com/lochness.php
Long article, this is just the beginning:A few reasons why the Loch Ness Monster is not a plesiosaur:Loch Ness can't support a population of large carnivorous animals
Loch Ness is a cold, deep lake, and has steeply sloping shores. This means that most of the water is cold and dark, and does not support much biological activity. If the Loch Ness monster is a plesiosaur, plesiosaurs would have to have survived for at least 65 million years. This could only happen if there was a substantial number of animals which would have formed a population large enough to avoid the problems of inbreeding. Loch Ness is neither big enough or productive enough to support such a population.
We would see them come up for air:
Although plesiosaurs lived in the water, they were air breathing reptiles. Even if they could submerge for a long time - marine turtles may be able to remain submerged for as long as five hours - they would still have to surface several times a day.We would find the bones:
If there is a breeding colony of plesiosaurs in Loch Ness, some will die. Usually when a large animal dies, its carcase sinks to the bottom of the lake or sea. Bacteria in the guts of the animal generate gas, and after a while it floats back to the surface. After a while, the gasses escape and the carcase sinks to the bottom again, usually rather disintegrated. If there were plesiosaurs in Loch Ness, the floating carcases would occasionally be seen, and would sometimes be washed up on the shore. On the other hand, it is claimed that the waters of the loch are so cold that this fermentation is delayed until the carcase has disintegrated. If this is the case, the loch is probably too cold for a colony of plesiosaurs. Carcases decaying on the shore would leave bones behind, and if plesiosaurs have been living in Loch Ness for a long time, there should be plenty of bones to be found.Loch Ness is too cold:
Reptiles do not generate their internal body heat, which is why they live in warmer climates. Marine reptiles in particular need to live in relatively warm water as they can't hibernate to survive cold winters. Large leatherback turtles are occasionally found further north than Scotland. A account (in Norwegian) of a leatherback found off the coast of northern Norway can be found here (thanks to Torfinn Ørmen for the information). However, this is presumably because they have been carried there by the Gulf Stream, which on the eastern side of the Atlantic is a warm, northerly current.