As I already mentioned:
But the reason the USCG requires ON-Scene SAR commanders to contact the shoreside SAR Command before boarding a foreign flag vessel is because…
A) it depends on more context (e.g. if it’s a slave ship or drug boat then you MUST board them)
B) the CO of the RCC has advanced training in the legality of birding ops and 24/7 access to USCG Admiralty lawyers.
I don’t mean to be vague on the topic but I have no choice… UNCLOS is intentionally vague, especially when it comes to government ships.