Lake monsters is a term that encompasses any mysterious creature suspected to be living in a lake. These creatures can be described as anything from a few metres to many metres long. They may be described as resembling prehistoric creatures, or they may be described as something supernatural.
In Scotland a lake is known as a loch, pronounced lock, and arguably the worlds most famous lake monster is the Scottish Loch Ness Monster. The Loch Ness Monster is affectionately known as Nessie by the locals.
Nessie is famous throughout the world, and has been investigated by several research teams using underwater cameras and sonar. Despite these attempts, Nessie has never been proven to exist.
Very weak at best. All known evidence of lake monsters can easily be disputed or explained with alternative hypotheses. There is an abundance of eyewitness testimony and blurry photographs or videos, but nothing significant enough to convince a skeptic. Even the one of the most well known pieces of photographic "evidence" known as "The Surgeons Photo" is now known to have been a hoax.
The evidence of absence on the part of Nessie, can be said to have strengthened over the past 30 years due to the number of high tech investigations that have turned up empty handed. The more we look for something without finding it, the less likely it is to have existed in the first place.
We don't dispute that eyewitnesses have seen something strange, but seeing something "strange" does not equate to "seeing a lake monster". If the human brain can't explain something, it has a tendency to fill in the blanks. There are countless possible explanations for the strange things people report seeing in or around lakes, and simply because there are a lot of sightings doesn't mean that any of them have to be a lake monster.
People overestimate their own memories and the clarity of their perception. For an idea of just how easy it is to be deceived check these videos out - Selective Attention Test, 10 Amazing Illusions and one of my personal favourites the Colour Changing Card Trick.
Further evidence suggesting that there are no lake monsters is that over many years these eyewitness descriptions tend to change based on social cues at the time of the sighting. If there were indeed a creature present, it is highly doubtful that the creature would be changing it's physical appearance over time.
Further adding to the evidence of absence of lake monsters is the fact that for a lake monster to exist for the length of time that some of these sightings have been occurring there would have to be not just one, but many of them. A minimum viable population would have to be present or the creature would die out fairly quickly. If such a minimum population did indeed exist, sightings would be far more prevalent and I'm sure we would have some clearer photographs by now.
The likelihood of a lake monster ever being shown to exist beyond any doubt is miniscule. It's not an impossibility, but it is near enough.