When you speculate about dinosaurs and humans co-existing, its probably a good idea to define
which dinosaurs co-existed with humans. "Dinosaur" covers an incredibly diverse range of creatures of many sizes and types, which lived over a very long period of time - any given dinosaur didn't even co-exist with many other dinosaurs!
Here, it seems you are mostly talking about the classic big dinosaurs. Tyrannosaurs, stegosaurus, diplodocus, plesiosaur, etc. The cool attention grabbing ones.
what if it is like this fish, thought to be extinct since the time of dinosaurs, but then found alive with very minor evolution
http://www.independent.co.uk/news/science/the-living-fossil-coelacanth-fish-left-behind-by-evolution-8577129.html
And here's where there's a big problem with thinking that the big cool dinosaurs could be around today, just like the coelacanth.
They are big and cool. There's a temptation to imagine some Hollywood style expedition to a remote part of the ocean discovering the long-lost coelacanth, but it was nothing like that. Local fishers caught them regularly - and generally threw them back because apparently they taste terrible. You see, by and large, a coelacanth looks pretty much like a
fish.
People were seeing and interacting with coelacanths. They just didn't know there was anything special or unusual about them, because honestly nobody without a specific background would be likely to know that.
But dinosaurs, like the tyrannosaurus and diplodocus? They are NOT fish. They are much bigger than most animals we see. They look radically different from most animals we see. Nobody is going to see a tyrannosaur and think "Oh, I guess its just a big bird and nothing interesting". Even if the viewer has no clue what a dinosaur is, they will damn well know its SOMETHING unusual.
And by virtue of people being pretty much everywhere, if there were big dinosaurs around, we'd have spotted them by now.
As for Loch Ness...people have been looking for a monster there for close to a hundred years now, and have nothing to show for it but blurry photos and exposed fakes. As more and more people carry cameras everywhere and camera quality improves, we don't get any better photos. There's no monster. Luke thought it was maybe a giant catfish...I think even that is putting too much substance to it. Like the "supernatural" everywhere, Nessie is easily explainable by pareidolia and wishful thinking.
Here's an interesting interview with the world's most dedicated Nessie-hunter. Even he dismisses most of the claims and sightings.
http://www.csicop.org/sb/show/alone_on_the_loch_one_mans_search_for_nessie/