This study explores the profound maritime dimension in Southeast Asia’s past and present. It highlights the region’s “sea people”, giving special attention to their complex and dynamic interactions with terrestrial communities. Their cultures and aquatic lifestyles, it is argued, are relevant to understanding the wider context of various historical and contemporary issues in this region. Since the emergence of large-scale harbour polities in Sumatra, Borneo and other parts of insular Southeast Asia, nomadic maritime communities have intermittently appropriated the roles of protégés, outlaws and victims in a multilayered and multifaceted interplay of seaborne navigation, commerce and warfare. On the one hand, many of the interethnic power relations that helped shape the maritime history of Southeast Asia have remained intact to date and can help us understand a number of present-day phenomena. On the other, 19th and 20th-century developments have largely confined these communities to the margins of society, while their erstwhile “maritory” has become the battleground of competing “globalised” crime syndicates, maritime terrorists and post-independent nation states with modern navies. The seascapes of Southeast Asia are as dynamic as they are diverse.