Read this very carefully and heed the advice: do not fly from Canada to Cuba.

Our "friends" at DHS require the manifests for flights that cross US airspace, even if the flight doesn't land in the United States. If they see US passports on a YYZ-HAV flight, they can easily forward your information to Treasury which may slap you with a stiff fine.

If you go (and not that I'd know anything about this from personal experience, of course) go from Cancun.