(1) The provision that may be made by virtue of section 17(2) (enforcement of prohibitions or requirements) includes provision as to the powers and duties of prescribed persons in relation to -

(a) British ships in foreign waters or international waters,

(b) ships without nationality in international waters, and

(c) foreign ships in international waters.

(2) Regulations may make provision by virtue of this section only for the purpose of enforcing relevant prohibitions or requirements.

(3) A prohibition or requirement is a "relevant prohibition or requirement" for the purposes of this section if it is -