A Northern Ireland constable may exercise the powers set out in Part 3 of Schedule 4A ("Part 3 powers") in relation to any of the following in Northern Ireland waters—
(a) a United Kingdom ship;
(b) a ship without nationality;
(c) a foreign ship;
(d) a ship registered under the law of a relevant territory.
But Part 3 powers may be exercised only—
(a) for the purpose of preventing, detecting, investigating or prosecuting an offence under
(b) in accordance with the rest of this section.
The authority of the Secretary of State is required before ... a Northern Ireland constable ... may exercise Part 3 powers in relation to a foreign ship, or a ship registered under the law of a relevant territory, within the territorial sea adjacent to the United Kingdom.
Authority for the purposes of subsection (3) may be given in relation to a foreign ship only if the Convention permits the exercise of Part 3 powers in relation to the ship.