I think the OP says it all. 
Seriously though.
According to the American Heritage Dictionary of the English Language, the etymology of canuck (“Often Offensive Slang A Canadian, especially a French Canadian”) is “probably alteration of Canadian”.
Word Detective says it’s probably from “Canadian.” Another theory has it that it’s a blend of “Canadian” and “inuk,” the singular of “inuit.” Yet another threory says that it’s Hawaiian.
The OED’s first citation is from 1835 when it was spelled “Kanuk.”
I’ve heard this definition before, which apparently stems from Nixon using it as a derogatory word about people from Vermont. I’m quite amused that the primary definition would be as an offensive one; would we really have a team called the Vancouver Canucks if it was offensive? Think “Yankee” without the civil war baggage. I can’t guarantee that it wouldn’t be offensive to any Canadian (I’m thinking here of hard-core separatists) but in the Rest of Canada it’s a self-applied term of endearment, although context always matters.
Some more theories:
The German racial insult “Kanacke” (meaning dark skinned foreigners in general) derives from the Hawaiian root mentioned above.