Canoeing in calm waters is peaceful and serene like when I attended the clinic but does absolutely nothing in preparing you for rapids even small ones. Our guide made note before the canoes hit the water in saying "It's not a matter of if you end up in the water but when". We did just that several times.
Second piece of advice would be to bring a small first aid kit. You will get a little banged up if you do come out of your canoe. I ended up with a couple of quarter size scrapes on my kneecap. One person in our group ended up chipping a tooth!
My last piece of advice is if it looks to difficult or you don't know what you should do in a certain area if you can, pick your canoe up and carry it past the difficult section or you can end up like us being the only one to lose our canoe permanently!