That's next to impossible. The bed of Loch Ness was ground out by glacial ice and it is composed of granite. Caves aren't really formed in granite; they require softer stones like limestone.

I would like to believe in Nessie, but I don't think it's possible. First of all, it would take a sizable population just to maintain breeding needs. And then, the amount of fish it would take to feed that many Nessies would be enormous or they would have to be bigger fish; more than can be sustained within Loch Ness's eco-system. Also, there have been no remains found ever. It's just impossible.