Prior to the United Nations Convention on the Law of the Sea, British officials in the Persian/Arabian Gulf seemed to be out of their depths regarding the law of the sea. Could concessions for pearls be granted by local rulers? Where did the high seas begin and end? Like other parts of the global ocean, custom was the driving force behind imperial policy over the pearl fishery, followed closely by the precedent set by other British officials. Toward the mid-20th century, however, custom began to shift abroad and British judges and men-on-the-spot wrestled with the coming tide of concessions for subsea oil. This talk looks at changing notions of the high seas and a dramatic reversal of policy in the imperial Gulf between the 19th and 20th centuries.