Ok I'll be the boring one and post a (semi) realistic answer.
There is precident in most cultures for belief in pre-historic or mythological beasts living in large lakes, so Nessie is far from unique. I do believe that there originally was *something* there (remember that people used to be scathing of folk tales of hairy elephants until someone fell over a mammoth's remains). Certainly the descriptions are all very similar, which implies their validity. I doubt though that any such creatures are alive today, as with all the interest in finding them one would have turned up somewhere by now. That doesn't detract however from the fun I used to have scouring Loch Ness for the beastie.