Bro, if Lock ness was full of baseball stadiums, you can fit the entire human population of 7 billion and way more, having a breading population is not a problem whatsoever for elusive creatures.
We are talking cubic dimension here not square area, huge difference in capacity.
Doesnt matter. Even if you ask marine biologist and zoologists about that. You need to think about this.
Predatory animals, large ones at that. How long do they live? What type of species fish are in the lake? Temp of the lake, how often do the species of fish breed? And what do they feed on? Then you have to ask about a breeding population. Are they inbreed? How did they get there? Are they warm blooded (chances are yes they are).
Just because you put 7 billion people in there, that lake couldn't support 1 million people in terms of fish stock. I would even say a far less amount of people would severely put a dent in fish stock...depending on what type of fish they have in there. As was stated, Loch Ness is Oligotrophic and is not that productive.
Now, let's take a look. A predatory animal that is at the least 20 feet long, and at the most 50 feet long and several tons in weight is going need a lot of food to sustain it. Now, how about a breeding population?
So, if that lake is not really productive, then let's say that they feed off of the benthos, that means that there has to be a lot of bottom dwelling animals down there, which is doubtful, as again, the lake is not that productive.
Everything has pointed to Sturgeon...photos flippers, etc. No real evidence of a plesiosaur. Would I like it to a population of plesiosaur? Hell Yes! But, it ain't practical.