This is what we call a sweeps stunt. The Nov. 6 episode of NCIS (CBS, 8 p.m.) begins with Gibbs (Mark Harmon, pictured) trapped in a submerged car with an unconscious young woman, and Tony (Michael Weatherly) diving into the water to save them. Of course, how Gibbs and the girl came to be sitting on the floor of the Washington Channel is a mystery that unfolds throughout the episode. But we were able to get NCIS stunt coordinator Diamond Farnsworth to tell us how the actors shot the memorable scene. In an 18-foot deep tank. Sucking air out of hookah lines. Read on…