It's not impossible that Loch Ness has provided a "Lost World" environment isolated from the rest of the world, but if there were plasiosaurs in Loch Ness, they wouldn't have been isolated until the last Ice Age, many millions of years after the K/T boundary. This would suggest that plesiosaurs would be found from a lot more recently than they have been. Also, it is likely that the creature would be highly specified and highly evolved compared to Mesozoic plesiosaurs. So, personally, I am deeply sceptical.
There you go. The first and last time I'm going to use my Palaeontology degree...