No legal requirement for barrel length WRT waterfowl hunting.
The frirearm in question must fit within the parameters of a non restricted firearm as perscribed in the Firearms Act.
See here:
http://www.cfc-cafc.gc.ca/faq/default_e.asp#a3
There is a upper gauge limit, there are non toxic shot requirements, but I've never heard of a minimum barrel lenght, outside of the Federal firearms classifications.