Law of amity and reciprocity

Via Hostis humanis generis:

A law of amity and reciprocity holds among the seafaring powers, especially in regard to matters related to the protection of life and to a lesser extent, property; for instance, the law stipulates the obligation of every mariner to assist those who are shipwrecked, and the obligation of every harbormaster to provide safe harbor to any vessel in need during a storm, regardless of the flag it flies.