Maritime law in fact allows nations to arrest foreign vessels for certain kind of claims, or maritime liens, and the cargo dispute between Iran and Maersk qualifies. However, the arrest of ships transiting international status is limited under the United Convention of the Law of the Sea, and general custom, to a limited set of claims involving the vessel itself. As UNCLOS Art. 28 states:
The coastal State may not levy execution against or arrest the ship for the purpose of any civil proceedings, save only in respect of obligations or liabilities assumed or incurred by the ship itself in the course or for the purpose of its voyage through the waters of the coastal State.
This reflects the in rem nature of admiralty and maritime proceedings. In short, there is simply no basis under UNCLOS for arresting a ship for any debts incurred during prior voyages, and certainly not for those incurred by other ships.
There is a broader and somewhat complicated doctrine that allows for the arrest of “sister ships,” that is ones that are owned by the same person that owns the ship against which the lien has been levied. This kind of cross-attachment is permitted by the 1952 International Convention Relating to the Arrest of Seagoing Ships (“Brussels Convention”), but neither Iran nor the Marshalls are parties (nor are they parties to the slightly more permissive 1999 updating of that treaty, which has only has 11 parties.)
The “sister ship” rule can be difficult to apply in an era when ownership, chartering and operation of ships are routinely separated, and ownership itself is often through single-ship companies in turn owned by other companies. But under the Brussel Convention, the “sister ship” concept is defined narrowly, applying only “when all the shares therein are owned by the same person or persons.” Art. 3(2).
Iran’s claims clearly do not satisfy any version of the “sister ship” rule. The Tigris is owned by Oak Tree Capital, and merely chartered by Maersk. The Iranian court judgement is apparently against Maersk itself, regarding the operation of a vessel with totally different owners. Instead, they are at most “associated ships.” Such vessels cannot be arrested under the Brussels Convention, or apparently under Iranian maritime law. To be sure, many countries now apply modern corporate veil-piercing doctrines to allow the attachment of vessels with a common “real” owner, but again, the common party here appears to only be the time-charterer.
The discussion of sister ships is quite arcane, and ultimately besides the point. The arrest of vessels applies typically to those that have come into port. There is no right to arrest for prior or collateral obligations during an passage through territorial waters. That is, in waters where ships have a right of transit passage through straits – such as the Gulf – they may not be molested at all by coastal states for the enforcement of outstanding maritime liens.
Thus the ownership of ships, the scope of the sister ship/associated ship doctrine, and so forth are entirely besides the point. Iran’s seizure clearly violates international law, and one might add, a branch of international law that is ordinarily well-respected, and quite fundamental for global commerce. Moreover, no maritime lien gives Iran any authority to detain the crew.
Given the flagrant breach of international law, there seems to be a surprising silence from the “international community” and proponents of global governance. Recall that Russia’s unlawful seizure of a Greenpeace vessel in 2013 lead to an outcry from NGOs, Nobelists, and numerous governments. To be sure, Arctic Horizon incident was more aggravated in some sense (the absurd piracy charges against the crew), but the Tigris incident is more aggravated in other ways (the Maersk ship was not engaged in any arguably illegal or dangerous activities when arrested, unlike the Greenpeace vessels).