While I cannot trace the origin of this account, it seems to be a fairly regularly told story. In 1880 Duncan McDonald, a diver, encountered a huge froglike animal in the water of the lake. It doesn't appear to have shown any hostility, but it must have been a startling experience.
In more modern times, silvery creatures that looked like mice were videotaped and photographed on the bed of Loch Ness. An expert at the British Museum (Natural History) was unable to identify the creatures.
Long before the Loch Ness Monster sightings of the 1930s, the lake was renowned for its huge eels. Could sightings of these account for the famous monster?