Jeremy Wade, host of the TV show "River Monsters", has an episode where he investigates the Loch Ness Monster and concludes it's likely a Greenland Shark that swam up an underground river from the North Atlantic to the lake. He likens the shark's horse-like face and the distribution of the low fins on the shark's back to descriptions of the monster. A solitary long-living fish could explain the occasional sightings, and the scientists' findings that there is not enough food in the lake for a breeding population of large carnivores.

As a sibling comment notes most sharks cannot live long in freshwater, and moreover this is soecifically true of Greenland sharks, though they do sometimes spend time in brackish river mouth environments, so, unless it developed the weird behavior of migrating quickly up the relevant underground river to make a quick appearance and then inmediately rushing back down the river to the ocean, that’s one answer we can be fairly certain is wrong.

There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_

He is likely wrong (most sharks cannot live long in fresh water). But given the show, he has to conclude it is a fish of some sort, and it is not going to be 10k arctic char in a dinosaur suit.

Doubt a shark could survive in freshwater. They’re very tuned to salinity

The most likely explanation for the Loch Ness Monster, of course, is that it's entirely made up and didn't require an actual sighting or a real physical phenomenon, ever, to trigger people's imaginations.