Naomi Watts stars in “The Impossible,” focusing on the 2004 tsunami, despite her fear of water after a near-drowning experience. “I got caught in a hell of a riptide in Bali . . . I wasn’t a strong swimmer,” Watts said at a screening of the flick on Wednesday. “Every time I . . . was about to say, ‘Please, no more [filming],’ I remembered I’m just an actor; I know I’m safe.” Petra Nemcova, who lost her fiancé during the tsunami, told us, “It’s very much reliving everything . . . the first half of the movie is 90 percent what I went through.”