If I consistently paddled cl III (or, shudder, higher), I would bag the canoe. Usually I encounter little more than low cl II, and since the canoe is generally full of gear, there isn't much space left for water. Most of my paddling is moving to swift water, with the occassional cl I.
As far as wearing a PDF, ALWAYS!!! I even wear it on the acre pond at the farm. It's a positive example for my grandkids. In fact, I just ordered a new Stohlquist eSCAPE this afternoon. I know it's the economy model, but it has to be at least as good as the Stearns that has serviced me so well for more years than one should trust any PFD. If I was really cheap, I'd have gotten the cheap orange yokes @ _ _ _ _ mart.
(BTW, has anyone figured out by now that I like smiley faces?)