For a loch ness monster type creature to exist it would truly need to be supernatural.
If there were only one it would need to be practically ageless and have survived since these creatures roamed the earth millions of years ago.
Otherwise you’ll have to have at least two creatures in Loch Ness for the entire time to maintain a lineage throughout that time, making it twice as likely to have been discovered
If you follow the rules of genetics the creature would likely be horribly deformed and riddled with genetic diseases if that were the case.
To avoid that you would probably need a population of 50-100 creatures within the loch ness in order to sustain a somewhat viable population.
It’s very unlikely that with how much attention has gone on, 50-100+ of those creatures would go undiscovered, let alone the sustenance needed to sustain that size of population.
So no, unless it’s a single supernatural creature it’s pretty much impossible I would say.