obviously this isn't official advice, but are they misinformed and don't like you doing legal things that bother them?

> Any small pleasure vessel leaving a United States port into international or foreign waters, without a call at a foreign port, does not satisfy the foreign departure requirement. Therefore, certain fishing vessels, cruises to nowhere, or any vessel that leaves from a United States port and returns without calling a foreign port or place, has not departed the United States.

Dunno how much emphasis to put on the word "port"

https://www.cbp.gov/travel/pleasure-boats-private-flyers/ple...

While it's a more inland water, Large vessels effectively can't transit through Windows/Detroit shipping channel without multiple border line crossings: https://i0.wp.com/niche-canada.org/wp-content/uploads/2020/1...