It's like any of these things - you need to weigh up the evidence. There is some shaky photographic and anecdotal evidence that Nessie exists, and quite a lot of other evidence that she doesn't, such as the 600-probe sonar sweep which showed nothing.
But ultimately nobody can "prove" a negative, so we can't be 100% sure that she doesn't. We can just be sensibly convinced by the evidence.
Sorry for a serious answer!!