I had a friend swim out of his kayak at about 19K above Big Hummer. Since he didn't have a solid roll, I told him to stay near the raft; which he did - but they just sat there and watched him float downstream as I screamed at them to throw him a rope. I ended up chasing him almost the whole way through the canyon, with him grabbing the stern of my boat (a Pyranha Storm, which was a pretty small boat). I couldn't get him into an eddy because of the huge eddy lines, and we kept getting pulled into the next rapid and I'd get flipped trying to pull him to the sides, and he'd have to let go. I finally got him to shore in the scout eddy at Skull - a mile or so downstream - and he was too exhausted to pull himself onto the rock. I was fully exausted too and we both just clung to the side until another paddler caught up with us.
Moral of the story: Westwater is no joke at higher flows. That was close as I've ever seen to a flush drowning- if I hadn't gotten him into that eddy, I think I wouldn't have had the energy to get him until after Last Chance, and that would have been too late. As a rafter, I'd be very hesitant to take anyone on my raft that hadn't swam Class IV, and they'd need a hi-float PFD and a good wetsuit or a drysuit. Think about trying to catch a swimmer once your boat got hooked by one of those massive eddy fences...
So please be careful.