What's not a sighting
Here's a list of common mistakes when it comes to seeing 'monsters' in Loch Ness or anywhere else:
Boat wakes - these can leave standing waves on the water long after the boat has gone, giving the impression that there are humps moving through the water. This is one of the reasons that people say that flat calm conditions lead to perfect 'Nessie spotting' weather as boat wakes are much more prominent on such days. This phenomenon is exacerbated in long narrow waterways like Loch Ness as the wakes bounce off the shores in calm weather and come out to meet each other again, causing even more humps.
Seals - yes, they end up in Loch Ness and other inland waterways looking for food. A famous sighting in 1998 was subsequently thought to be a seal.
Otters - smaller and rarer than seals but still around causing confusion in a similar manner to seals.
Deer - they have been known to go for a swim and be mistaken for Nessie - horns can sometimes be a bit of a giveaway though.
Logs and other floating debris - especially after heavy rainfall when they can be washed into the loch. To see a good example of this (and one that a daily paper supposedly paid for) click here
Birds - birds landing and taking off from the water can cause a wake. Dick Raynor, a long time investigator, took footage of a duck when he was with the LNIB which perfectly illustrates this. Also, long necked birds can cause an optical illusion in the distance, looking bigger than they actually are. Furthermore, birds flying past a camera lens can look like something in the water when viewed from some angles.
Boats - boats (and canoes in particular) can be mistaken for something animate, especially when photographed from a distance.
Flies - yes, you might not believe it but flies and other insects crawling or landing on a camera lens just as a picture is taken can look very large and scary, hence being mistaken for Nessie.
Divers - in March 2010 Connie Ross saw what she thought was Nessie and the story made the national newspapers. Unfortunately, she'd seen divers working for Google Maps who were including Loch Ness on their streetview service. Just goes to show that there are all sorts in the loch.....
The best way to ensure than none of the above are what you have seen is to take a photo - this allows proper analysis to be undertaken of what's been seen. One of the reasons that the number of sightings listed has gone down over the last decade is the advent and spread of digital photography - almost everyone can take an instant picture or footage of something they see. This means that the common mistakes can be discounted very quickly - in fact reported sightings haven't dropped but unknown and therefore listed ones have reduced due to pictures having been taken whereas 20 years ago, the technology wasn't around to take them .
You'll find a good example of what's not a sighting here