A more thorough explanation:

A sea letter is a document that was issued during a war by authorities of a port where a neutral vessel is fitted. It certifies the vessel's nationality, specifies the nature of and destination of the vessel's cargo, and allows the vessel to sail under the neutral flag of its owner. The last sea letter was issued at the Port of New York in 1806, and the use of sea letters was discontinued by proclamation of President James Madison.

For example, if a ship from a neutral country like Sweden wanted to trade with a country at war like France, it would need a sea letter to prove that it was not carrying any goods that could help France in the war. The sea letter would also allow the ship to sail under the Swedish flag instead of being stopped and searched by French warships.

Our laws require masters of vessels, on entering a port for traffic, to lodge with the consul their registers, sea-letters, and passports ....” Theodore D. Woolsey, Introduction to the Study of International Law 161–62 (5th ed. 1878).