Which document must you always request during a boarding?

Requesting the vessel's Certificate of Documentation or state registration is fundamental during a boarding because it serves as an official record that proves the vessel’s nationality and ownership. This document allows the boarding officer to verify that the vessel is compliant with U.S. laws and regulations, as well as international maritime law. It provides crucial information regarding the vessel's registration details, including the hull identification number and the name of the registered owner.

Additionally, having the Certificate of Documentation or state registration helps ensure that the vessel is not operating illegally. This document often contains critical information that supports further inspections of compliance with safety standards, environmental regulations, and other maritime laws that the Coast Guard is responsible for enforcing.

While the other documents mentioned might also be relevant and useful during the boarding process, they do not hold the same primary importance as the vessel's registration. The captain's personal identification, a list of crew members, and insurance papers can provide supplementary information but are secondary to validating the vessel's legitimacy and regulatory compliance through its Certificate of Documentation or state registration.