Leaving a U.S. port to anchor in Canadian waters does not constitute leaving the United States or require reporting to Customs and Border Protection (CBP) on return to port.
CBP's Assistant Port Director in Port Huron recently clarified their policy on "pleasure vessel reporting after anchoring in foreign waters". It reads as follows (with syntax modified for clarity):
"[U.S. flagged] pleasure vessels are only required to report their arrival when they have touched foreign soil, have had contact with a foreign hovering vessel or received merchandise outside United States territorial waters. Small pleasure vessels leaving [a] United States port [and proceeding] into international or foreign waters without a call at a foreign port are not considered to have made a departure for the purpose of CBP reporting requirements. Therefore. . .any vessel which leaves from [a] United States port and returns without calling at a foreign port or place [has] not departed the United States and [is] not required to report [its] arrival to CBP."
This interpretation is slightly at odds with Canadian policy, in that they maintain that anchoring anywhere in Canadian waters constitutes landing in that country.