GI Joe, I'm also a flatwater kayaker, and save the whitewater for guided trips. My previous experience before doing the Pacuare was oar-boats and J-rigs on the Colorado river (Westwater and Cataract canyons), so I was a bit nervous about paddle rafts, but it turned out fine. Rafts have cross-tubes in which you jam your feet under (people in front also have a floor pocket) to anchor yourself, plus the paddle also helps to brace yourself. Two trips already and I haven't fallen out. Anyone can do it, on my first trip the group I was included an old guy and a 20-something girl.

On the last trip the guide actually had us get down into the bottom of the raft during several of the bigger rapids; less exciting but I guess more safe.
The river is very scenic, going through virgin rain forest and two canyons where all the big rapids are located; the second of which also has a very narrow but flatwater section. Lots of class IIs and IIIs before you get the IVs, most of which are quite visible and steep drops.
The trip is usually done in one day, you get back in time for evening activities.

It also can be done as an overnighter, though the river distance you go done is the same.