I never worried about bouncing out of the Aqua Duck, but I did worry about getting stuck and not knowing what to do next I know they tell you to stay there and wait...but I wouldn't like to sit there and wait... haha. However, never stopped me from riding it.
Now.. I know we are talking about the Aqua Duck - but I do have a similar worry about Summit Plummet at Disney's Blizzard Beach. I know it's off topic... but how does one not fall out of there? Have never been on it but DS9 announced he really wants to try it next time we go. Meanwhile I have visions of him flying off the slide... I know it won't happen, but yeah... bit paranoid.
THe forces involved on Summit Plummet very effectively shove you INTO the slide. There is a high probability you will get a wedgie. I find anywhere bare skin touches the slide ends up quite irritated from the friction. Your head and feet bounce (painfully) some, but the rest of you is shoved into the slide very thoroughly. And then you come to a sudden and full stop.
Disney uses quality rides: the physics are well designed. Guests won't be unintentionally flying out of them.
SW