You need to be comfortable floating in water while wearing a lifejacket and you need the ability to propel yourself through the water to assist in your own rescue should you fall out of the raft. Falling off the boat into the river, or having your boat capsize is one of the inherent risks associated with whitewater rafting. If this happens, you will need the ability to self-rescue by swimming to the boat or to shore.
Each guest wears a lifejacket and the lifejackets, if worn properly, are very effective at keeping you above water, but if you are unable to swim, it can still be a threatening situation because the waters are turbulent.