A big problem I have with the Nessie theories is that there have never been any remains found. Sure, it might be a bigger problem that no one can actually find a creature, either, but let's say she's just really good at hiding, and isn't hundreds of years old, so surely we would have seen remains?
But sharks, you see, have skeletons made from cartilage. So they decompose in a way that bony creatures just wouldn't, which is why megalodon (the really big shark) is such a find, because I think all they've ever found is teeth. So if Nessie's skeleton was cartilaganous, it would make a bit more sense as to why we wouldn't have found any skeletal remains.
Plus, sharks don't have gas-filled swim bladders to keep them afloat, so (as I understand it) their remains are less likely to float to the surface when they die. AND - recently it came to light that the Greenland shark can live for hundreds of years. So that does set a precedent for just one, really old creature. Still not sure she could live to over 1500 though.