The watershed of Loch Ness covers more than 700 square miles (1,800 square km) and comprises several rivers, including the Oich and the Enrick. Its outlet is the River Ness, which flows into the Moray Firth at Inverness. Seiches (seiche) (surface oscillations), caused by differential heating, are common on the loch. The sharp rise and fall of the level of the loch is one reason for the scanty flora of the waters; another reason is the great depths of the loch near the shoreline. The abyssal fauna is also sparse.

Like some other very deep lochs in Scotland and Scandinavia, Loch Ness is said to be inhabited by an aquatic monster. Many sightings of the so-called Loch Ness monster have been reported, and the possibility of its existence—perhaps in the form of a solitary survivor of the long-extinct plesiosaurs—continues to intrigue many.