I must admit I don't quite understand the logic. There are species around which are very closely related to dinosaurs as I understand it. Turtles in particular (if my understanding is correct) have been around since what most people consider dinosaurs were around (around 200 million years ago), so would be a far better example than a creature for which any evidence is tenuous at best.
Even if Nessie does exist, and is a plesiosaur, that doesn't prove or disprove evolution, it just means that evidence for that particular species extinction was incorrect. There have been plenty of other species which have been believed to have been extinct, but were later found alive, the Coelacanth being one of the classic example (which itself was probably around 400 million years ago).