This paper engages in a microhistory of international law, grounded in the contests surrounding the Muscat Dhows case brought by Great Britain against France in 1905. At the heart of the case was the question of whether the French consul had the right to grant flags and navigation passes to dhows from the southern Omani port of Sur that were suspected of transporting slaves. The case became foundational to studies of the law of the sea, and the ruling is still cited in footnotes in law school textbooks. Buried in the case's proceedings, however, are a series of petitions by the dhow captains that give historians a window into the legal imaginaries of Indian Ocean mariners in an age of empire. Through a close reading of the petitions, I explore how captains located themselves within an imperial legal geography, and appropriated legal technologies—passes and flags—to help them shape the legal possibilities of a changing political and economic seascape. I argue that the claims the captains articulated and the practices they engaged in at sea reveal a maritime legal culture at work, one animated by a long history of encountering regional and global empires at sea. Their documentary practices illuminate how they engaged in and domesticated a body of international law, and illustrate how the regime manifested itself in an ocean that ran thick with legal idioms.