Maritime security capacity-building is a growing field of international activity. It is an area that requires further study, as a field in its own right, but also as an archetype to develop insights for capacity-building and security sector reform in other arenas. This article is one of the first to analyse this field of activity. Our empirical focus is on the Western Indian Ocean (WIO) region. Here, international actors have launched multiple capacity-building projects, initially in response to Somali piracy. We document the significance, extent and variety of capacity-building activities in this region and examine the ways in which capacity-building at sea has incorporated innovative characteristics that develop and expand the capacity-building agenda as traditionally understood. Our conclusion highlights the need to pay more attention to the maritime domain in international security and development studies and considers ways in which the maritime capacity-building experience may offer important lessons for other fields of international policy.