The most plausible explanation for Nessie as real is that it is a mutant eel that never matured and kept growing. Although rare, such eels can become quite large. Specimens might appear sporadically, reside in Loch Ness for a time, and then die off or migrate to the sea. This would account for the intermittent nature of Nessie sightings and for similar reports from other fresh water bodies of water.
Or a separate species of large eel with a small population. Either way, those explanations may not be as sexy as a plesiosaur, but are more plausible.