Tidewater Cave is a cave overlooking the Topal Bay in Southern Cyrodiil on the opposite bank south of Leyawiin. It plays a role in Nocturnal's quest, as it is the place where the Eye of Nocturnal can be retrieved.
Nocturnal speaks for a time about her Eye of Nocturnal, then informs the Hero that it has been stolen. She tasks them to find out where the thieves have hidden it, and return it. In passing she makes reference to a pair of Argonians, Weebam-Na and Bejeen, in the nearby town.