What a cool thread! Never trained a dog to be a canoeing partner, but I
think I'd also suggest getting her used to her PFD beforehand. Maybe put it
on her, then go play frisbee or something so she gets used to PFD = fun.
Smack her if she chews on it. I don't know if they make commercial doggie
PFDs, but I'd look into designs....I imagine you'd want more flotation down
around her chest area to keep her head up, but not so wide that it
interferes with her dogpaddling.

An example of a good doggie PFD

http://www.pooch.ca/item1140.htm

a lot of cheaper vest just have straps going around the stomach instead of
the wrap around float panel, making them uncomfortable to wear and don't
offer proper flotation.