From May to October 2017, the city of Marawi in the Philippines was the scene of an intense urban battle, as government security forces fought to retake control of the city from militants claiming affiliation with the Islamic State (also known as ISIS/ISIL). This six-month battle was the “longest urban war” in Philippine history. One reason for the length of the fight had to do with geography; Marawi is located along the shore of Lake Lanao. Without the maritime access provided by the lake, the militants would not have been able to continue fighting for as long as they did. This paper examines the importance of the maritime domain to violent extremist organizations (VEOs), and argues that VEOs develop maritime capabilities when maritime access offers strategic utility to the group and when the state does not have adequate maritime capabilities to counter VEO use of waterways. The first section examines the specific ways in which maritime capability might offer VEOs strategic benefits. Then, the paper shifts to looking at a case study of maritime VEO operations. Drawing on the theoretical discussion of the strategic utility of the maritime domain, we examine how the Islamic State movement and its global affiliates found operational uses for the maritime domain. The chapter conclude with an analysis and observations on how this case can help us understand the current militant use of the maritime domain.