Some scientists argue that a giant aquatic reptile would have a hard time surviving in the frigid waters of the Scottish lake (around 5℃, on average). But sea turtles are large reptiles as well (some exceed 200kg) and they can thrive in the cold waters off the coasts of Iceland and Alaska.
A study by French scientist Aurélien Bernard and colleagues, published in Science in 2010, examined the oxygen isotopes extracted from the fossil bones of plesiosaurs. The isotopes showed that these animals were likely capable of adjusting their body temperature (endothermic), and were therefore capable of surviving in relatively cold waters.
However, unless Nessie is a fish, it would need to surface regularly in order to breath and so its apparitions should be fairly frequent. But they are not.