A good reference to grasp the authenticity of a similar situation (large river flowing through a canyon, lack of PFD’s, minimal/lack of technology) is J.W. Powell’s geological report on the Colorado River and his 1869 expedition: “The Exploration of the Colorado River and Its Canyons.”
I’d recommend Chapter VII, The Canyon of Lodore, which gives an account of one of the expedition’s river running issues (wooden boat flipping, men swimming/holding onto rocks without PFD). It’s a geological report, so it also does a good job of describing the river’s features, how those features change in a rapid, and the effect those features had on the expedition’s boats. This section of the Green River is a popular spot for modern day white water trips (commonly referred to as the Gates of Lodore), so you can reference plenty of youtube videos for a better idea of what precipitates. This chapter will also show you why a group of boys swimming in a large white water rapid with no flotation and still surviving is unlikely.