In “Finding Nemo,” Pixar’s 2003 masterpiece, the ocean was a vast realm of menace and wonder, newly charted by rapidly advancing digital-animation technology. The movie, a visual revelation, was also a welcome defense of risk-taking in an era of anxiety, and something of a cautionary tale about the downsides of helicopter parenting. As often happens in adventure stories, the hero was occasionally upstaged by his sidekick. We rooted for Nemo and choked up when he was found, but the best lines and sweetest grace notes belonged to Dory, the absent-minded blue tang voiced by Ellen DeGeneres.