This chapter explores the problems which union posed to maritime lawyers of both nations. It approaches the questions from an oblique angle by asking why in one area it did become possible to speak with some plausibility of a body of British law. In particular, it tries to address why lawyers in Scotland began to accept between the Unions of 1603 and 1707 that their sea law might need to be exposed to influence from south of the border. It then considers why the handling of maritime matters excepted from the general insistence on the preservation of a separate legal system in Scotland. A theoretical difference between approving the customs of mariners and determining the requirements of international law is vital, for it displays that the move towards developing a British law of the sea cannot simply have led from recognition that many maritime disputes had to be determined in accordance with international law. To secure the coasts against invasion and to promote mutual trade, there had to be a British admiralty regulating the exercise of jurisdiction over British ships in the British seas.