Why do some people believe that the loch ness monster is actually a plesiosaur?

The arguments against the existance of the loch ness monster as a plesiOSaur are plentiful.For example,a plesiOSaur could not live in the frigid waters of loch ness.A single plesiOSaur living in loch ness for centuries?Ridiculous.It would have to be a breeding population of them and to prevent inbreeding you would need a HUGE population.Loch Ness couldn't possibly support such a large population, size wise alone.They would need a food supply and the loch couldn't possibly hold that many fish.A large population of plesiOSaurs would have easily been spotted by now as they would have to come up for air frequently.Some suggest tunnels that lead from the loch to the ocean and these creatures regularly go back and forth.If that was the case,the loch's water would be the same level as the Atlantic Ocean.That's not the case.The vast majority of marine creatures can only survive in salt or fresh water.A salmon can live in both types of water,but that's about the only marine animal that can do so.That's one in thousands of water dwelling species.Salmon are just about the only exception to this rule.A baby whale got stuck in the Delaware river somehow and people were scrambling for a way to get it back to sea, as the fresh water was harming the whale's skin.Also, a plesiOSaur carcass has never washed up on shore.With that many dinosaurs living in a lake,that would definitely have happened by now.