As much as I would love it to be fact I would guess its myth. Unless its a serious freek of nature and been alive as a single animal for millions of years it would need a breeding population and in that case there would be more than 1 or 2 around, therefore the detailed research with sonnar etc should have produced more evidence than it has. One possible explanation is that the breeding population is located out in the sea and the sightings in Loch Ness are just the odd creature popping in to the loch for some reason. I conclusion I think that it is possible for a type of creature to have survived this long but not isolated in a single place.
I think it is a myth as I find it impossible to believe that there could be something there after all the research into the Loch. I would also love it to be true.
Possibly a fact since dinosaurs died out during and after the global flood about seven thousand years ago. So it is very possible and even probable that some sea dinosaurs survived. no doubt places with caverns, deep waters and access to the sea via caves would have been good hidig places. At least before the tourists scared the away.
The whole story was long ago exposed as a hoax. However, who knows? There might, unbeknown to anybody, really be a monster so the original hoaxers were wasting their time in creating the story !