There is no way you will sink with the life jackets they give you for the white water rafting.
They are alot like the ones we muster in on ship. No chance of swimming in the rapids, if you should happen to fall overboard, you would just float down to the next calm spot and get pulled aboard there. ( I only saw one person go over on our trip and there were about 150 people in about 35 rafts) The water is warm too, not like north american white water. What you need to be in shape for is not so much the paddeling that is mainly for steering a direction, it's for the hike down to the river and back up once you are taken out of the river, I'd say it's about a mile of steep rocky terrain you hike from where the bus can go no further. Each raft has it's own guide and you are given a full safety and instruction briefing on the bus ride to the river. This excursion is something I definately want to do again. The guides have the best job in the world, beautiful country, and excitement every day. Photographers with 6.5 megapixel cameras paddle ahead of the rafts on kayaks and photograph the whole trip. We bought the CD after the trip with 350 incredible photos for $30.