An immigration officer or an enforcement officer may exercise the powers set out in Part A1 of Schedule 4A ("Part A1 powers") in relation to any of the following in United Kingdom waters, foreign waters or international waters—
a United Kingdom ship;
a ship without nationality;
a foreign ship;
a ship registered under the law of a relevant territory.
But Part A1 powers may be exercised only—
for the purpose of preventing, detecting, investigating or prosecuting a relevant offence, and
in accordance with the rest of this section.
The authority of the Secretary of State is required before an immigration officer or an enforcement officer may exercise Part A1 powers in relation to—
a United Kingdom ship in foreign waters,
a ship without nationality,
a foreign ship, or
a ship registered under the law of a relevant territory.