So, has the Nessie question finally been solved? It is/was a giant eel?
European eels only grow to 1m long. The Moray eel is bigger, but still not big enough. However, the Slender Giant Moray or Gangetic moray, Strophidon sathete, is the longest member of the family of moray eels. The longest recorded specimen was caught in 1927 on the Maroochy River in Queensland; it measured 3.94 metres.
As far as I remember, there are locks on the Caledonian Canal at either end of Loch Ness. The juvenile eels could swim in and grow to adulthood, but the locks would prevent anything that long gaining exit, at least unseen. Once inside it would be stuck in Loch Ness. It would be unable to swim back into the ocean to spawn and die.
They can, however, live for years in freshwater, growing bigger and bigger. Once it had eaten all the fish, hungry and cold, it would come up to the foreshore of the loch and snatch away lambs and little babies. [doubtful]
So, they should build special Nessie eel ladders parallel to the locks. I'd subscribe to the fund.