Well, the under the sea ending is one valid ending, and I would have been perfectly happy with the film if it ended there. That's not quite my problem with it, though. I kind of liked the 'aliens' (who I felt were clearly just robots created by robots created by robots, over thousands of generations) in general... I didn't need them, but I thought they had a place in the film. My problem was with the whole out of the blue, contrived, 'but only for a single day!' crap. If they ended the movie saying they couldn't bring her back, fine. If they ended it bringing her back, fine... maybe not as poignant, but I can enjoy that as a happy ending that works fine. Hell, even if they ended it with the advanced mechas creating a robo-Mommy for David, deluding him but keeping him happy. But the nonsensical 'only for a single day, and then never again' thing bugged the crap out of me. It comes out of nowhere and its only purpose is to give the exact ending they wanted for the movie, even though it didn't flow from the rest of the plot.