This article discusses the changing spirit world of maritime communities in Southeast Asia by differentiating ‘oceans’ from ‘seas’ and by linking historical evidence to modern anthropological studies. Since the lives of seagoing peoples are fraught with unpredictability, propitiation of local sea spirits was a traditional means of ensuring good fortune and protection. As long-distance voyages expanded in the early modern period, the global reach of the world religions, extending beyond familiar seas into the more extensive ocean environment, held out particular appeal. Not only were the gods, deities and saints attached to larger religious systems themselves ocean travellers; in contrast to the unpredictability of indigenous spirits, they were always amenable to requests for help, even when the suppliant was far from home waters. At the same time, as world religions were incorporated into indigenous cosmologies, maritime peoples gained greater agency in negotiating relationships with the local spirits that still wield power in Southeast Asian seas.