I've often wondered about the possibility that these lake monsters are some large unknown species of amphibians. Having fished freshwater lakes, ponds, rivers, streams and creeks, I've seen different species of amphibians that breathe under water with gills that thrive in colder waters.
What if such supposed creatures like Nessie were huge fully aquatic amphibians ? Being water breathers, they would not be sighted all that much as there would be little need to surface unless hunting near the surface, or moving through shallow waters.