The limit on boarding is contained in Article 110 of the UN Convention on the Law of the Sea
Right of visit
- Except where acts of interference derive from powers conferred by
treaty, a warship which encounters on the high seas a foreign ship, other than
a ship entitled to complete immunity in accordance with articles 95 and 96,
is not justified in boarding it unless there is reasonable ground for suspecting
(a) the ship is engaged in piracy;
(b) the ship is engaged in the slave trade;
(c) the ship is engaged in unauthorized broadcasting and the flag
State of the warship has jurisdiction under article 109;
(d) the ship is without nationality; or
(e) though flying a foreign flag or refusing to show its flag, the ship
is, in reality, of the same nationality as the warship.
In the cases provided for in paragraph 1, the warship may proceed
to verify the ship's right to fly its flag. To this end, it may send a boat under
the command of an officer to the suspected ship. If suspicion remains after
the documents have been checked, it may proceed to a further examination on
board the ship, which must be carried out with all possible consideration.
If the suspicions prove to be unfounded, and provided that the ship
boarded has not committed any act justifying them, it shall be compensated
for any loss or damage that may have been sustained.
These provisions apply mutatis mutandis to military aircraft.
These provisions also apply to any other duly authorized ships or
aircraft clearly marked and identifiable as being on government service.
COLREG requires that all vessels avoid collisions; there is no right-of-way in navigation. A warship is generally more maneuverable than a merchant vessel and therefore has a greater ability (and responsibility) to avoid collisions. However, a warship executing a war-like function against a non-complying vessel also has more leeway if a collision occurs.
Of course, Article 95 of UNCLA gives a warship complete immunity other than in their home state for acts on the High Seas, so an Iranian merchant ship that wished to pursue damages would need to do so in the US Admiralty Courts.