As a kayaker graciously invited on a GC trip a few years back, the joke was that the yak-trash would be upstream surfing just when they were needed. In reality, the two kayaks would run first on anything class 6 and up (Desert scale 1-10) and then wait in the wash-out eddy below. Something to remember is that the GC water is REALLY COLD! And, obviously, big. A swimmer could be in the water a long time before making it to shore on their own. We did pull three people out (one injured) and chased, and secured, the flipped raft.
It's a good precaution to take a kayak or two, if you have room. Look for someone with a larger volume river running boat, not a low volume playboat. And make sure your kayakers have a really solid roll, or you will be saving them, not the other way around.
Kayakers are just like rafters; some good, some not. Make it clear ahead of time that you expect them to run safety, and tell them that if it wasn't for you, they would be sitting on the couch at home, waiting for their unemployment check; so you expect them to bust their sorry a$$e$ hauling gear, cooking, and breaking down the groover, just like everybody else!