The flag state of a merchant vessel is the jurisdiction under whose laws the vessel is registered or licensed, and is deemed the nationality of the vessel. A merchant vessel must be registered and can only be registered in one jurisdiction, but may change the register in which it is registered. The flag state has the authority and responsibility to enforce regulations over vessels registered under its flag, including those relating to inspection, certification, and issuance of safety and pollution prevention documents. As a ship operates under the laws of its flag state, these laws are applicable if the ship is involved in an admiralty case.

Currently, the Aquarius is temporarily registered by the Liberian Maritime Authority.