I think the six reasons below for doubting the existence of a monster in the loch make a lot of sense.
http://www.plesiosaur.com/lochness.php
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. 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.
The eye-witness accounts of the Loch Ness monster are nothing like a plesiosaur:
There are many different descriptions of the Loch Ness monster, and not all of them agree with each other. Typical descriptions talk of a series of undulating humps. This would imply that the observers are looking at a mammal - reptiles can only move their bodies from side to side. The bodies of plesiosaurs were rigid, and could certainly not undulate either up and down or from side to side. Some descriptions refer to a neck held vertically out of the water. The bones of the neck, especially at its base connect in such a way that a plesiosaur could not hold its neck straight up. Other accounts talk of a animal walking on the shore of the loch. Marine turtles come onto land only to lay eggs, and find it very hard work. Plesiosaurs were less suited than turtles to move about on land.
They couldn't have survived in Loch Ness:
Loch Ness is only 10,000 years old. Plesiosaurs became extinct 65 million years ago.