Back in the mid-1980s the Discovery Channe'l, back when they took themselves seriously, conducted an expedition at the loch, and put together some actual scientist-types.
The best part was they broke down all of the sightings into type:
1-Long neck/small head.
2 - Full body.
3 - One hump.
4 - Two humps.
5 - Three or more humps.
6 - Other.
Their graph indicated most people reported one hump cruising at the surface. From there that particular scientist suggested tossing out the other types of sightings based on the statistics, and the fact that those other sightings, especially the long neck were both rare, and easily debunked. This left a hump that averaged three to four feet in length (est). This allowed them to establish a base from which they could identify alternate creatures to explain the sightings. They suggested seals, otters, and eels.
Personally I think if there's something in the loch, it's a large fresh water salamander. They can grow to 6 feet in length, have the same red-brown skin color, and their gills allow them to remain submerged. They certainly have the ability to hide. And some look like eels.