Falling out of the raft during whitewater rafting is actually pretty normal; it's far from risk free, but as long as you curl up you're probably safe. The rapids come in waves, so in between sections of rapids there's plenty of calm water to get back in the raft.

A few of my friends in middle school actually had their raft guide fall out of the raft mid-rapid. He shouted "get down," but they thought he said "get out," so they all jumped out too. Imagining that poor raft guide's face as six 13 year-olds abandon ship for no reason never fails to crack me up. Everyone was fine haha.