Some years ago when the state stopped sending boat tab renewal notifications, I let ours slip by. Went to Desolation for two months and came back with the intent to telephone clear at Semi-ah-moo where we kept our boat.
I had just crossed the border through Georgia Strait when a fast Customs boat approached along side with a Skagit County Sheriff deputy and Border Customs Agent aboard. The Deputy asked to come aboard, which I obliged. He went through my papers and ask several questions. At that point he asked why I had expired boat tabs. News to me.
He cited me for expired tabs, explained the citation fee ($300), and said that if I appeared in court, often times the judge will offer a reduced fine. So on my court date I appeared before the judge with a current boat tab and pleaded my case. The judge then turned to the prosecutor and asked for remedy. The prosecutor said that they would settle for court cost of $100, Whew! Lesson learned. Of course that fine was far better than the tab fee of over $700 at the time.
I have never had a Canada Border agent ask about boat tabs. They seem only concerned with a valid Passport and any illegal fresh foods, excessive alcohol, or firearms.