The scene where Rapunzel and Flynn are trapped in the sealed cave with rapidly-rising water, thinking they're about to die. Partly because of Rapunzel's tearful apologizing and "Mother was right; I never should have left..." and partly because of Flynn trying to tell her it's hopeless a few seconds earlier.
Flynn: "It's no use! I can't see anything!"
*Rapunzel panics for a second, then dives underwater*
Flynn: "Hey! Hey!" *pulls her out, holding her shoulders* "There's no point!'' It's pitch-black down there..."