In response to another post, do not use a car seat to secure the baby in your canoe!! If the car seat should become dislodged, the life jacket wouldn't be bouyant enough to keep the baby and the seat afloat. Surely, the seat could sink, and if you're not quick, you may not be able to catch the baby before she goes down, let alone get her out of the straps in time. If the canoe tips, and the seat remains in tact, she could be trapped and held face down under water until help arrives, or you are able to get her released from her seat.
Just remember, they (infant floatation devices) are designed for the weight of a child, not a child and something else. Also, most infant vests are designed to keep a baby's head above water, not their entire body.
As for where to put your child, and how should you secure her in a canoe, I'd strongly recommend contacting the State Department of Natural Resources or your local water patrol office (sheriff's department water unit) and talk with them about their recommendations on infant water safety in a canoe.
If you're in the Twin Cities, I know folks at Hennepin County water patrol (offices off lake Minnetonka) would be very happy to take the time to talk about which type of floatation devices you should use, and what you should or should not do in case the boat tips. They'd probably be able to give you a recommended age range for safe/appropriate water safety with a really young child. Perhaps if you're elsewhere, you'd want to call your local sheriff's department.
In short, get your advice for this one from the experts!!