It seems to me that existence of a bigfoot is more likely than a Nessie since men and various apes are known to exist on the planet now while no dinosaurs are known to be alive today, especially in a lake that is known to have been under a mile or two of glacier during the last Ice Age.
In fact, there does seem to be some super gorilla recently discovered in Africa that is just a bit bigger and a bit more aggressive than other known gorillas. But that is just another species of gorilla in a remote location, not an ape man running about undetected in a highly developed country.